Optical hybrid circuit, optical receiver and light receiving method

ABSTRACT

An optical hybrid circuit includes a MMI coupler including a pair of input channels provided at positions symmetrical with respect to a center position in a widthwise direction thereof, a pair of first output channels outputting a pair of first optical signals having an in-phase relationship, and a pair of second output channels neighboring with each other outputting a pair of second optical signals having an in-phase relationship. The MMI coupler converts QPSK signal light or DQPSK signal light into the pair of first optical signals and the pair of second optical signals having an in-phase relationship. The optical hybrid circuit includes a 2:2 optical coupler connected to the first or the second output channels. The 2:2 optical coupler converts the pair of first optical signals or the pair of second optical signals into a pair of third optical signals having a quadrature phase relationship with the pair of first or second optical signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of thetwo prior Japanese Patent Applications No. 2008-333753, filed on Dec.26, 2008 and No. 2009-140362 filed on Jun. 11, 2009, the entire contentsof which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical hybridcircuit, an optical receiver and a light receiving method.

BACKGROUND

In recent years, in order to increase the transmission capacity of anoptical transmission system, an optical transmission system having ahigh bit rate higher than 40 Gbit/s has been and is being researched anddeveloped.

In order to cope with rapid increase of the network traffic, furtherincrease of the bit rate is essentially required. Particularly as anoptical modulation system which achieves optical transmission of 50Gbit/s or more, a quadrature phase shift keying (QPSK) system or adifferential quadrature phase shift keying (DQPSK) system is consideredmost promising.

In order to demodulate signal light modulated by such a QPSK system or aDQPSK system as just described, a coherent optical receiver including a90-degree hybrid is required. Here, the 90-degree hybrid exhibits outputforms (patterns) having a different branching radio depending upon thephase modulation state of QPSK signal light or DQPSK signal light and isthe most important component of a coherent optical receiver.

As conditions demanded for such a 90-degree hybrid as just described,low loss, a wide band characteristic property of an operating wavelength(low wavelength dependency), a low phase displacement characteristic,compactness, a monolithic integration characteristic and so forth can belisted.

At present, 90-degree hybrids which use a bulk component are placed onthe market.

FIG. 48A is a view illustrating a general configuration of a 90-degreehybrid which uses a bulk component, and FIG. 48B is a phase relationshipdiagram illustrating a phase relationship of optical signals outputtedfrom the 90-degree hybrid.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 48A indicate what relative relationship the phase of localoscillation (LO) light (L) has with reference to the phase of signallight (S). Here, S−L and S+L indicate that they have a phaserelationship displaced by 180 degrees from each other, and S+jL and S−jLindicate that they have a phase relationship displaced by 90 degreeswith respect to S+L and S−L, respectively. Further, the phaserelationship diagram of FIG. 48B illustrates a phase relationship ofoptical signals outputted from the 90-degree hybrid in response to arelative phase difference between the QPSK signal light and the LOlight.

As seen in FIG. 48A, QPSK signal light and LO light are inputted to twoinput channels of the 90-degree hybrid. Then, optical signals having anin-phase relationship with each other are outputted from a first outputchannel (Ch-1) and a second output channel (Ch-2) from among four outputchannels. Meanwhile, optical signals having a quadrature phaserelationship with the optical signals having the in-phase relationshipare outputted from a third output channel (Ch-3) and a fourth outputchannel (Ch-4) from among the four output channels of the 90-degreehybrid.

Such a 90-degree hybrid formed using a bulk component as just describedhas such superior characteristics as low loss, a low wavelengthdependency and a low phase deviation characteristic.

Meanwhile, also a 90-degree hybrid having an optical waveguide structurewhich can be monolithically integrated has been and is being researchedand developed.

FIGS. 49A and 50A illustrate general configurations of 90-degree hybridsbased on waveguide optics, and FIGS. 49B and 50B are phase relationshipdiagrams illustrating phase relationships of optical signals outputtedfrom the 90-degree hybrids of FIGS. 49A and 50A, respectively.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL ineach of FIGS. 49A and 50A indicate what relative relationship the phaseof LO light (L) has with reference to the phase of signal light (S).Here, S−L and S+L indicate that they have a phase relationship displacedby 180 degrees from each other, and S+jL and S−jL have a phaserelationship displaced by 90 degrees with respect to S+L and S−L,respectively. Further, the phase relationship diagram of each of FIGS.49B and 50B illustrates a phase relationship of optical signalsoutputted from the 90-degree hybrid in response to a relative phasedifference between the QPSK signal light and the LO light.

First, the 90-degree hybrid illustrated in FIG. 49A is formed from four3-dB couplers and a 90-degree phase shifter. A phase relationship ofoptical signals outputted from the 90-degree hybrid formed in thismanner is similar to that of the 90-degree hybrid formed using a bulkcomponent described hereinabove as illustrated in FIG. 49B.

The 90-degree hybrid having such a configuration as described above issuitable for monolithic integration and is expected to have a lowwavelength dependency and a low phase displacement characteristic.

On the other hand, the 90-degree hybrid illustrated in FIG. 50A isformed from a 4:4 multimode interference (MMI) coupler having fourchannels on both of the input side and the output side thereof.

Here, in order to obtain 90-degree hybrid operation using a 4:4 MMIcoupler, it is necessary to select two channels at asymmetricalpositions from among four channels on the input side of the 4:4 MMIcoupler as input channels for inputting QPSK signal light and LO light.With such selection, a relationship of phases different from each otherby 90 degrees is obtained inevitably by mode interference in the MMIregion of the 4:4 MMI coupler, and therefore, the 4:4 MMI coupler can beused as a 90-degree hybrid.

The 90-degree hybrid having such a configuration as described above issuitable for monolithic integration and is superior in that it can beconfigured compact.

However, a phase relationship of optical signals outputted from the90-degree hybrid just described indicates rotation by approximately 45degrees with respect to the phase relationships [refer to FIGS. 48B and49B] of the 90-degree hybrids illustrated in FIGS. 48A and 49A asillustrated in FIG. 50B. This is because, when two input lightsinterfere in mode with each other, a phase difference by 45 degrees isproduced inevitably.

Meanwhile, in the case of the 90-degree hybrid illustrated in FIG. 50A,a pair of optical signals having an in-phase relationship with eachother are outputted from the two outer side channels (Ch-1 and Ch-4)while a pair of optical signal having a quadrature phase relationshipwith the pair of optical signals having the in-phase relationship areoutputted from the two inner side channels (Ch-2 and Ch-3). In short, apair of optical signals having an in-phase relationship with each otherare outputted from two output channels (Ch-1 and Ch-4) spatially spacedaway from each other.

SUMMARY

According to an aspect of the embodiment, an optical hybrid circuitincludes a multimode interference coupler including a pair of inputchannels provided at positions symmetrical with each other with respectto a center position in a widthwise direction thereof, a pair of firstoutput channels neighboring with each other for outputting a pair offirst optical signals having an in-phase relationship with each other,and a pair of second output channels neighboring with each other foroutputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other, and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals.

According to another aspect of the embodiment, an optical hybrid circuitincludes a multimode interference coupler including a pair of inputchannels provided at positions symmetrical with each other with respectto a center position in a widthwise direction thereof, a pair of firstoutput channels neighboring with each other for outputting a pair offirst optical signals having an in-phase relationship with each other,and a pair of second output channels neighboring with each other foroutputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other, and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals, the multimode interference coupler having an input end of afirst width and an output end of a second width different from the firstwidth such that a phase difference between the pair of first opticalsignals or a phase difference between the pair of second optical signalsbecomes equal to π/2+p*π, p being an integer.

According to a further aspect of the embodiment, an optical receiverincludes a multimode interference coupler including a pair of inputchannels provided at positions symmetrical with each other with respectto a center position in a widthwise direction thereof, a pair of firstoutput channels neighboring with each other for outputting a pair offirst optical signals having an in-phase relationship with each other,and a pair of second output channels neighboring with each other foroutputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other, and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals, a photodiode adapted to convert the first optical signals orthe second optical signals outputted from the multimode interferencecoupler and the third optical signals outputted from the 2:2 opticalcoupler into an analog electric signal, an analog-digital conversioncircuit adapted to convert the analog electric signal outputted from thephotodiode into a digital electric signal, and a digital arithmeticcircuit adapted to execute a arithmetic processing using the digitalelectric signal outputted from the analog-digital conversion circuit.

According to a still further aspect of the embodiment, an opticalreceiver includes an optical hybrid circuit including a multimodeinterference coupler including a pair of input channels provided atpositions symmetrical with each other with respect to a center positionin a widthwise direction thereof, a pair of first output channelsneighboring with each other for outputting a pair of first opticalsignals having an in-phase relationship with each other, and a pair ofsecond output channels neighboring with each other for outputting a pairof second optical signals having an in-phase relationship with eachother, the multimode interference coupler being adapted to convertquadrature phase shift keying signal light or differential quadraturephase shift keying signal light into the pair of first optical signalshaving an in-phase relationship with each other and the pair of secondoptical signals having an in-phase relationship with each other, and a2:2 optical coupler connected to the first output channels or the secondoutput channels and having two channels on the input side and twochannels on the output side, the 2:2 optical coupler being adapted toconvert the pair of first optical signals or the pair of second opticalsignals into a pair of third optical signals having a quadrature phaserelationship with the pair of first or second optical signals, themultimode interference coupler having an input end of a first width andan output end of a second width different from the first width such thata phase difference between the pair of first optical signals or a phasedifference between the pair of second optical signals becomes equal toπ/2+p*π, p being an integer, a photodiode adapted to convert the firstoptical signals or the second optical signals outputted from themultimode interference coupler and the third optical signals outputtedfrom the 2:2 optical coupler into an analog electric signal, ananalog-digital conversion circuit adapted to convert the analog electricsignal outputted from the photodiode into a digital electric signal, anda digital arithmetic circuit adapted to execute arithmetic processingusing the digital electric signal outputted from the analog-digitalconversion circuit.

According to a yet further aspect of the embodiment, a light receivingmethod includes converting, using a multimode interference couplerincluding a pair of input channels provided at positions symmetricalwith each other with respect to a center position in a widthwisedirection thereof, a pair of first output channels neighboring with eachother for outputting a pair of first optical signals having an in-phaserelationship with each other, and a pair of second output channelsneighboring with each other for outputting a pair of second opticalsignals having an in-phase relationship with each other, quadraturephase shift keying signal light or differential quadrature phase shiftkeying signal light into the pair of first optical signals having anin-phase relationship with each other and the pair of second opticalsignals having an in-phase relationship with each other, converting,using a 2:2 optical coupler connected to the first output channels orthe second output channels and having two channels on the input side andtwo channels on the output side, the pair of first optical signals orthe pair of second optical signals into a pair of third optical signalshaving a quadrature phase relationship with the pair of first or secondoptical signals, and receiving the first optical signals or the secondoptical signals and the third optical signals.

According to a yet further aspect of the embodiment, a light receivingmethod includes converting, using a multimode interference couplerincluding a pair of input channels provided at positions symmetricalwith each other with respect to a center position in a widthwisedirection thereof, a pair of first output channels neighboring with eachother for outputting a pair of first optical signals having an in-phaserelationship with each other, and a pair of second output channelsneighboring with each other for outputting a pair of second opticalsignals having an in-phase relationship with each other, quadraturephase shift keying signal light or differential quadrature phase shiftkeying signal light into the pair of first optical signals having anin-phase relationship with each other and the pair of second opticalsignals having an in-phase relationship with each other, converting,using a 2:2 optical coupler connected to the first output channels orthe second output channels and having two channels on the input side andtwo channels on the output side, the pair of first optical signals orthe pair of second optical signals into a pair of third optical signalshaving a quadrature phase relationship with the pair of first or secondoptical signals, receiving the first optical signals or the secondoptical signals and the third optical signals, the multimodeinterference coupler having an input end of a first width and an outputend of a second width different from the first width such that a phasedifference between the pair of first optical signals or a phasedifference between the pair of second optical signals becomes equal toπ/2+p*π, p being an integer.

The object and advantages of the embodiment will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view illustrating a configuration of an opticalhybrid circuit according to a first embodiment, and FIG. 1B is a phaserelationship diagram illustrating a phase relationship of lightsoutputted from each of channels of the optical hybrid circuitillustrated in FIG. 1A;

FIG. 2A is a schematic view illustrating action by a 2:4 MMI couplerwhich composes the optical hybrid circuit according to the firstembodiment, and FIG. 2B is a phase relationship diagram illustrating aphase relationship of lights outputted from each of channels of the 2:4MMI coupler shown in FIG. 2A;

FIG. 3A is a schematic view illustrating operation by the 2:4 MMIcoupler and a 2:2 MMI coupler which compose the optical hybrid circuitaccording to the first embodiment, and FIG. 3B is a phase relationshipdiagram illustrating a phase relationship of lights outputted from eachof channels of the 2:4 MMI coupler and the 2:2 MMI coupler illustratedin FIG. 3A;

FIG. 4 is a view illustrating a subject of the optical hybrid circuithaving such a configuration as illustrated in FIG. 3A and illustrating arelative output intensity (Transmittance) of the 90-degree hybrid withrespect to Δφ;

FIG. 5 is a schematic sectional view illustrating a configuration of anoptical semiconductor device which composes the optical hybrid circuitaccording to the first embodiment;

FIG. 6 is a schematic view illustrating an example of a particularconfiguration of the 2:4 MMI coupler which composes the optical hybridcircuit according to the first embodiment;

FIGS. 7A and 7B are schematic views illustrating an example of aparticular configuration of the 2:2 MMI coupler which composes theoptical hybrid circuit according to the first embodiment;

FIG. 8 is a schematic plan view illustrating a configuration of a phaseshifter which composes the optical hybrid circuit according to the firstembodiment;

FIGS. 9A to 9D are views illustrating input-output characteristics whereQPSK signal light (Signal) and LO light are inputted to the opticalsemiconductor device which composes the optical hybrid circuit accordingto the first embodiment, and wherein FIG. 9A illustrates an input-outputcharacteristic where Δφ=0, FIG. 9B illustrates an input-outputcharacteristic where Δφ=π, FIG. 9C illustrates an input-outputcharacteristic where Δφ=−π/2, and FIG. 9D illustrates an input-outputcharacteristic where Δφ=π/2;

FIG. 10A is a view illustrating a relative output intensity(Transmittance) of the 90-degree hybrid which uses a 4:4 MMI couplerwith respect to Δφ, and FIG. 10B is a view illustrating a relativeoutput intensity (Transmittance) of the 90-degree hybrid according tothe first embodiment with respect to Δφ;

FIG. 11 is a view illustrating a connection relationship between theoptical hybrid circuit according to the first embodiment andphotodiodes;

FIG. 12A is a view illustrating a wavelength dependency of thetransmittance of lights outputted from four output channels (Ch-1, Ch-2,Ch-3 and Ch-4) where the signal light is inputted to one input channelin an example of a configuration of a 90-degree hybrid which uses a 4:4MMI coupler, and FIG. 12B is a view illustrating a wavelength dependencyof the transmittance of lights outputted from four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light is inputted to oneinput channel in an example of the configuration of the 90-degree hybridaccording to the first embodiment;

FIG. 13A is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in an example of aconfiguration of a 90-degree hybrid which uses a 4:4 MMI coupler, andFIG. 13B is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in the example of theconfiguration of the 90-degree hybrid according to the first embodiment;

FIG. 14A is a view illustrating a wavelength dependency of thetransmittance of lights outputted from the four output channels (Ch-1,Ch-2, Ch-3 and Ch-4) where the signal light is inputted to one inputchannel in a different example of a configuration of a 90-degree hybridaccording to the first embodiment, and FIG. 14B is a view illustrating awavelength dependency of the transmittance of lights outputted from fouroutput channels (Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light isinputted to one input channel in a different example of theconfiguration of the 90-degree hybrid according to the first embodiment;

FIG. 15A is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in a different exampleof a configuration of a 90-degree hybrid which uses a 4:4 MMI coupler,and FIG. 15B is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in the different exampleof the configuration of the 90-degree hybrid according to the firstembodiment;

FIG. 16 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a modification to the first embodiment;

FIG. 17 is a schematic view illustrating a configuration of an opticalhybrid circuit according to another modification to the firstembodiment;

FIG. 18 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a further modification to the firstembodiment;

FIGS. 19A and 19B are schematic views illustrating a configuration of anoptical hybrid apparatus according to a still further modification tothe first embodiment;

FIGS. 20A and 20B are schematic views illustrating a configuration of anoptical hybrid apparatus according to a yet further modification to thefirst embodiment;

FIGS. 21A and 21B are schematic views illustrating a configuration of anoptical hybrid apparatus according to a yet further modification to thefirst embodiment;

FIG. 22A is a schematic view illustrating a configuration of an opticalhybrid circuit according to a second embodiment, and FIG. 22B is a phaserelationship diagram illustrating a phase relationship of lightsoutputted from each of channels of the optical hybrid circuitillustrated in FIG. 22A;

FIG. 23 is a conceptual view of an MMI waveguide;

FIG. 24 is a view illustrating a relationship between a ratioW_(M)/W_(S) between the width W_(S) of an input end and the width W_(M)of an output end of a 2:4 MMI coupler which composes the optical hybridcircuit according to the second embodiment and the absolute value |Δθ|of a phase difference between channels of output signals;

FIG. 25 is a view illustrating the ratio W_(M)/W_(S) between the widthW_(S) of the input end and the width W_(M) of the output end of the 2:4MMI coupler which composes the optical hybrid circuit and 1/χ^(ST)according to the second embodiment;

FIGS. 26A to 26D are views illustrating input-output characteristicswhere the QPSK signal light (Signal) and the LO light are inputted tothe optical semiconductor device which composes the optical hybridcircuit according to the second embodiment, and wherein FIG. 26Aillustrates an input-output characteristic where Δφ=0, FIG. 26Billustrates an input-output characteristic where Δφ=π, FIG. 26Cillustrates an input-output characteristic where Δφ=−π/2, and FIG. 26Dillustrates an input-output characteristic where Δφ=π/2;

FIG. 27A is a view illustrating a relative output intensity(Transmittance) of the 90-degree hybrid which uses a 4:4 MMI couplerwith respect to Δφ, and FIG. 27B is a view illustrating a relativeoutput intensity (Transmittance) of the 90-degree hybrid according tothe second embodiment with respect to Δφ;

FIG. 28 is a view illustrating a connection relationship between theoptical hybrid circuit according to the second embodiment andphotodiodes;

FIG. 29A is a view illustrating a wavelength dependency of thetransmittance of lights outputted from the four output channels (Ch-1,Ch-2, Ch-3 and Ch-4) where the signal light is inputted to one inputchannel in an example of a configuration of a 90-degree hybrid whichuses a 4:4 MMI coupler, and FIG. 29B is a view illustrating a wavelengthdependency of the transmittance of lights outputted from the four outputchannels (Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light is inputtedto one input channel in an example of the configuration of the 90-degreehybrid according to the second embodiment;

FIG. 30A is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in an example of aconfiguration of a 90-degree hybrid which uses a 4:4 MMI coupler, andFIG. 30B is a view illustrating a wavelength dependency of the phasedisplacement amount Δψ of lights outputted from the four output channels(Ch-1, Ch-2, Ch-3 and Ch-4) where the signal light and the LO light havean in-phase relationship with each other (Δφ=0) in the example of theconfiguration of the 90-degree hybrid according to the secondembodiment;

FIG. 31 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a modification to the second embodiment;

FIG. 32 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a different modification to the secondembodiment;

FIGS. 33A to 33D are views illustrating input-output characteristicswhere the QPSK signal light (Signal) and the LO light are inputted tothe optical semiconductor device which composes the optical hybridcircuit according to the different modification to the secondembodiment, and wherein FIG. 33A illustrates an input-outputcharacteristic where Δφ=0, FIG. 33B illustrates an input-outputcharacteristic where Δφ=π, FIG. 33C illustrates an input-outputcharacteristic where Δφ=−π/2, and FIG. 33D illustrates an input-outputcharacteristic where Δφ=π/2;

FIG. 34 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a further modification to the secondembodiment;

FIG. 35 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a still further modification to the secondembodiment;

FIG. 36 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a third embodiment;

FIG. 37 is a view illustrating a relationship between a ratioW_(M)/W_(S) between the width W_(S) of an input end and the width W_(M)of an output end of a 2:4 MMI coupler which composes the optical hybridcircuit according to the third embodiment and the absolute value |Δθ| ofa phase difference between channels of output signals;

FIG. 38 is a view illustrating the ratio W_(M)/W_(S) between the widthW_(S) of the input end and the width W_(M) of the output end of the 2:4MMI coupler which composes the optical hybrid circuit and 1/χ^(SQ)according to the third embodiment;

FIG. 39 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a fourth embodiment;

FIG. 40 is a view illustrating a relationship between a ratioW_(M)/W_(S) between the width W_(S) of an input end and the width W_(M)of an output end of a 2:4 MMI coupler which composes the optical hybridcircuit according to the fourth embodiment and the absolute value |Δθ|of a phase difference between channels of output signals;

FIG. 41 is a view illustrating the ratio W_(M)/W_(S) between the widthW_(S) of the input end and the width W_(M) of the output end of the 2:4MMI coupler which composes the optical hybrid circuit and 1/χ^(EXP)according to the fourth embodiment;

FIG. 42 is a schematic view illustrating a configuration of an opticalreceiver according to a fifth embodiment;

FIG. 43 is a schematic view illustrating a configuration of an opticalreceiver according to a modification to the fifth embodiment;

FIG. 44 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a sixth embodiment;

FIG. 45 is a schematic view illustrating a configuration of an opticalhybrid circuit according to a modification to the sixth embodiment;

FIG. 46 is a schematic view illustrating a configuration of an opticalreceiver according to a seventh embodiment;

FIG. 47 is a schematic view illustrating a configuration of an opticalreceiver according to a modification to the seventh embodiment;

FIG. 48A is a schematic view illustrating a configuration of aconventional 90-degree hybrid based on bulk optics and FIG. 48B is aphase relationship diagram illustrating a phase relationship of lightsoutputted from each of channels of the 90-degree hybrid illustrated inFIG. 48A;

FIG. 49A is a schematic view illustrating a configuration of a 90-degreehybrid which uses four 3-dB couplers and a phase shifter and FIG. 49B isa phase relationship diagram illustrating a phase relationship of lightsoutputted from each of channels of the 90-degree hybrid illustrated inFIG. 49A;

FIG. 50A is a schematic view illustrating a configuration of a 90-degreehybrid which uses a 4:4 MMI coupler and FIG. 50B is a phase relationshipdiagram illustrating a phase relationship of lights outputted from eachof channels of the 90-degree hybrid illustrated in FIG. 50A; and

FIG. 51 is a schematic view illustrating a connection relationshipbetween the 90-degree hybrid illustrated in FIG. 50A and photodiodes.

DESCRIPTION OF EMBODIMENTS

Since the 90-degree hybrid described hereinabove with reference to FIG.48A uses a bulk component, it has such demerits that it is not suitablefor monolithic integration, that it lacks compactness and that it ishigh in cost.

Meanwhile, since the 90-degree described hereinabove with reference toFIG. 49A requires four 3-dB couplers and a phase shifter, it includesmany components and has a demerit that it is complicated inconfiguration. Further, since it includes without fail a region in whichoptical waveguides intersect with each other, it has another demeritthat it involves excessive loss in the intersecting region.

Further, the 90-degree hybrid described hereinabove with reference toFIG. 50A has a demerit that it exhibits a conspicuous wavelengthdependency in comparison with the 90-degree hybrid described hereinabovewith reference to FIG. 49A. In other words, the 90-degree hybriddescribed hereinabove with reference to FIG. 50A has a demerit that ithas a comparatively high wavelength dependency.

Further, in the 90-degree hybrid described hereinabove with reference toFIG. 50A, where the output channels thereof and balanced photodiodes(BPD) are connected to each other to carry out optoelectronicconversion, optical waveguides for connecting them intersect with eachother as illustrated in FIG. 51. Therefore, the 90-degree hybriddescribed hereinabove with reference to FIG. 50A has demerits also thatit gives rise to excessive loss in the intersecting region and that itis poor in compatibility with a 90-degree hybrid used in coherentoptical receivers and so forth at present.

In particular, optical signals outputted from a 90-degree hybrid arenormally detected by BPDs as illustrated in FIG. 51 in order to carryout photoelectric conversion.

In the case of the 90-degree hybrid described hereinabove with referenceto FIG. 50A, it is necessary to connect two output channels (Ch-1 andCh-4) from which a pair of optical signals having an in-phaserelationship with each other are outputted to one BPD and connect twooutput channels (Ch-2 and Ch-3) from which a pair of optical signalshaving a quadrature phase relationship with the pair of optical signalshaving the in-phase relationship are outputted to the other BPD.

However, in the 90-degree hybrid described hereinabove with reference toFIG. 50A, a pair of optical signals having an in-phase relationship witheach other are outputted from two output channels (Ch-1 and Ch-4) whichare spatially spaced away from each other. Therefore, as illustrated inFIG. 51, optical waveguides which connect the two output channels (Ch-1and Ch-4) and one BPD to each other and optical waveguides which connectthe other two output channels (Ch-2 and Ch-3) and the other BPDinevitably intersect with each other. Accordingly, the 90-degree hybriddescribed hereinabove with reference to FIG. 50A has a demerit thatexcessive loss occurs in the intersecting region of the opticalwaveguides, which gives rise to deterioration of the receptionefficiency.

Also it seems a possible idea to connect output channels and BPDs suchthat an intersecting region does not appear while an electric wiringscheme of the BPDs is adjusted. Any schemes intersecting the electrodesare not preferred due to the reason of complicating the fabricationprocesses, as well as the reason of the electrode capacitance.

Therefore, it is desired to implement an optical hybrid circuit, anoptical receiver and a light receiving method which have a lowwavelength dependency, a low phase displacement characteristic and lowinsertion loss and are suitable for compactness and monolithicintegration.

In the following, an optical hybrid circuit, an optical receiver and anoptical transceiver and a light receiving method according toembodiments are described with reference to the drawings.

First Embodiment

First, an optical hybrid circuit according to a first embodiment isdescribed with reference to FIGS. 1A to 15B.

The optical hybrid circuit according to the present embodiment is a90-degree hybrid circuit (hereinafter referred to as 90-degree hybrid)used for identification (demodulation) of phase modulation informationof a quadrature phase shift keying (QPSK) signal in an opticaltransmission system (optical communication system).

In the present embodiment, as illustrated in FIG. 1A, the optical hybridcircuit 1 includes a multimode interference (MMI) coupler 2 at apreceding stage and an optical coupler 3 at a succeeding stage, whichare connected in cascade connection to each other. The optical hybridcircuit 1 is configured from an optical semiconductor device whichincludes the MMI coupler 2 and the optical coupler 3 and has asemiconductor waveguide structure.

Here, the MMI coupler 2 at the preceding stage is a 2:4 MMI couplerwhich has two channels on the input side and has four channels on theoutput side thereof.

More particularly, the MMI coupler 2 is a 2:4 MMI coupler based onpaired interference (PI). In other words, the MMI coupler 2 is a 2:4 MMIcoupler wherein the centers of the two input channels are positioned at⅓ and ⅔ from the upper side of the MMI width (refer to FIG. 6) andhigher-order modes of the (3s-1)th order (s is a natural number equal toor greater than 1) is not excited in the MMI region. Therefore, thedevice length can be reduced.

It is to be noted that, while a 2:4 MMI coupler based on PI is usedhere, the MMI coupler 2 is not limited to this, but a 2:4 MMI couplermay be used which has a structure having a center symmetric propertysuch that a pair of input channels are provided at symmetrical positionswith respect to the center portion in the widthwise direction. Forexample, a 2:4 MMI coupler based on general mode interference (GI:General Interference). In other words, a 2:4 MMI coupler may be usedwherein the centers of the two input channels are positioned withinregions except the positions of ⅓ and ⅔ of the MMI width within a rangewithin which the center symmetrical property of the MMI region is notlost and all modes according to the MMI width are excited.

The optical coupler 3 at the succeeding stage is a 2:2 optical couplerwhich has two channels on the input side thereof and has two channels onthe output side thereof and has a function of delaying the phase oflight, which propagates from the two input channels toward the twooutput channels positioned on diagonal lines, by 90 degrees.

In particular, the optical coupler 3 is a 2:2 MMI coupler. Here, the 2:2MMI coupler 3 is connected to the two channels on the output side of the2:4 MMI coupler 2 which are positioned on the third and the fourth fromabove (in other words, to a pair of second output channels neighboringwith each other). It is to be noted that the 2:2 MMI coupler 3 may bebased on PI or on GI.

Therefore, the optical hybrid circuit 1 has two channels on the inputside and has four channels (Ch-1, Ch-2, Ch-3 and Ch-4) on the outputside thereof.

To one of the channels on the input side of the optical hybrid circuit1, that is, to one of the channels of the input side of the 2:4 MMIcoupler 2, QPSK signal light is inputted. In other words, one of thechannels on the input side of the optical hybrid circuit 1 is an inputchannel for inputting QPSK signal light. Meanwhile, to the other channelon the input side of the optical hybrid circuit 1, that is, to the otherchannel on the input side of the 2:4 MMI coupler 2, local oscillation(LO) light is inputted. In other words, the other channel on the inputside of the optical hybrid circuit 1 is an input channel for inputtingLO light.

Then, as illustrated in FIG. 2A and FIG. 2B, the 2:4 MMI coupler 2converts the QPSK signal light into a pair of first optical signalshaving an in-phase relationship with each other and a pair of secondoptical signals having an in-phase relationship with each other. Inparticular, the QPSK signal light is converted into a pair of firstoptical signals which do not include a quadrature phase component(Q-component) but include only an in-phase component (I-component) and apair of second optical signals which do not include a quadrature phasecomponent (Q-component) but include only an in-phase component(I-component).

It is to be noted that S−L and S+L in FIG. 2A illustrate whatrelationship the phase of the LO light (L) has relatively with referenceto the phase of the signal light (S). Here, it is illustrated that S−Land S+L have a phase relationship displaced by 180 degrees from eachother. Meanwhile, the phase relationship diagram of FIG. 2B illustratesa phase relationship of optical signals outputted in response to therelative phase difference between the QPSK signal light and the LOlight.

Here, the pair of first optical signals are outputted from the twochannels on the output side of the 2:4 MMI coupler 2 which arepositioned on the first and the second from above (in other words, apair of first output channels neighboring with each other), that is,from the two channels (Ch-1 and Ch-2) on the output side of the opticalhybrid circuit 1 which are positioned on the first and the second fromabove. Meanwhile, the pair of second optical signals are outputted fromthe two channels on the output side of the 2:4 MMI coupler 2 which arepositioned on the third and the fourth from above (in other words, apair of second output channels neighboring with each other), andinputted to the two channels on the input side of the 2:2 MMI coupler 3which are positioned on the first and the second from above.

Then, as illustrated in FIGS. 3A and 3B, the pair of second opticalsignals are converted into a pair of third optical signals having aquadrature phase relationship with the pair of first optical signals bythe 2:2 MMI coupler 3. In other words, the pair of second opticalsignals which include only an in-phase component (I-component) areconverted into a pair of third optical signals which include only aquadrature phase components (Q-component).

Then, the pair of third optical signals are outputted from the twochannels on the output side of the 2:2 MMI coupler 3 which arepositioned on the first and second from above, that is, from the twochannels (Ch-3 and Ch-4) on the output side of the optical hybridcircuit 1 which are positioned on the third and the fourth from above.

The optical hybrid circuit 1 having such a configuration as describedabove outputs a pair of first optical signals (S−L and S+L) having anin-phase relationship with each other and a pair of third optical signal(S−jL and S+jL) having a quadrature phase relationship with the pair offirst optical signals as illustrated in FIGS. 3A and 3B.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 3A indicate what relative relationship the phase of the LO light(L) has with reference to the phase of the signal light (S). Here, S−Land S+L indicate that they have a phase relationship displaced by 180degrees from each other, and S+jL and S−jL indicate that they have aphase relationship displaced by 90 degrees with respect to S+L and S−L,respectively. Further, the phase relationship diagram of FIG. 3Billustrates a phase relationship of optical signals outputted from the90-degree hybrid in response to a relative phase difference between theQPSK signal light and the LO light.

In this manner, the output intensity ratio of the signal lightsoutputted from the four output channels (Ch-1, Ch-2, Ch-3 and Ch-4) ofthe optical hybrid circuit 1 differs depending upon the phase (0, π,−π/2 and +π/2) of the QPSK signal light.

The reason why the QPSK signal light is converted into first opticalsignals having an in-phase relationship with each other and secondoptical signals having an in-phase relationship with each other by the2:4 MMI coupler 2 and then the second optical signals are converted intothird optical signals having a quadrature phase relationship with thefirst optical signals by the 2:2 MMI coupler 3 as described above issuch as follows.

If QPSK signal light and LO light are inputted to the 2:4 MMI coupler 2as illustrated in FIG. 2A, then a pair of first optical signals havingan in-phase relationship with each other are outputted from two channelsof the 2:4 MMI coupler 2 and a pair of second optical signals having anin-phase relationship with each other are outputted from the other twochannels of the 2:4 MMI coupler 2.

Where the relative phase difference Δφ between the QPSK signal and theLO light is 0 and π, the intensity ratio among the four outputcomponents (output intensity ratio) is 0:2:2:0 and 2:0:0:2,respectively. In other words, where the relative phase difference Δφ is0 and π, output forms having different branching ratios from each othercan be obtained.

However, in both of the cases wherein the relative phase difference Δφis −π/2 and +π/2, the output intensity ratio is 1:1:1:1. In other words,where the relative phase difference Δφ is −π/2 and +π/2, an output formhaving an equal branching ratio is exhibited.

Therefore, as illustrated in the phase relationship diagram of FIG. 2B,the optical hybrid circuit 1 does not function as a 90-degree hybridalthough it functions as a 180-degree hybrid. Where a 2:4 MMI couplerhaving a center symmetric property, for example, like a 2:4 MMI couplerwhich is based on PI, it is impossible in principle to cause the 2:4 MMIcoupler to operate as a 90-degree hybrid.

Therefore, the 2:2 MMI coupler 3 is connected in cascade connection tothe 2:4 MMI coupler 2 having a structure having a center symmetricproperty as illustrated in FIG. 3A to form a structure having anasymmetric property so that the optical hybrid circuit 1 can function asa 90-degree hybrid.

In particular, the 2:2 MMI coupler 3 is connected in cascade connectionto the third and fourth output channels of the 2:4 MMI coupler 2 so thatonly output components of the third and fourth output channels of the2:4 MMI coupler 2 are subject to new phase shift together with acoupling action when they propagate in the 2:2 MMI coupler 3. Here, bythe provision of the 2:2 MMI coupler 3, output forms having differentbranching ratios can be obtained also where the relative phasedifference Δφ is −π/2 and +π/2 as illustrated in the phase relationshipdiagram of FIG. 3B. It is to be noted that a similar characteristic canbe obtained with the 2:2 MMI coupler 3 only if it is based on GI or PI.

Consequently, the optical hybrid circuit 1 outputs a pair of firstoptical signals (S−L, S+L) having an in-phase relationship with eachother and a pair of third optical signals (S−jL, S+jL) having aquadrature phase relationship with the pair of first optical signals asillustrated in FIG. 3A.

Here, FIG. 4 is a result of plotting of the output intensity ratio(relative output intensity; transmittance) with respect to the relativephase difference Δφ of the optical hybrid circuit having such aconfiguration as illustrated in FIG. 3A.

If the output intensity ratio to the relative phase difference Δφ isconverted into a linear value and compared, then it becomes 0:2:1:1(Δφ=0), 2:0:1:1 (Δφ=n), 1:1:1.7:0.3 (Δφ=−π/2) and 1:1:0.3:1.7 (Δφ=+π/2)as illustrated in FIG. 4. In particular, in comparison with where therelative phase difference Δφ is 0 or n, where the relative phasedifference Δφ is −π/2 or π/2, the branching ratios of the third andfourth output channels have a tendency that a high output component ofthe optical output power decreases and a low output component of theoptical output power increases. Therefore, crosstalk appears, and thecharacteristic deteriorates. However, where the relative phasedifference Δφ is 0, n, −π/2 and +π/2, since output forms havingdifferent branching ratios are obtained (that is, since a phasecondition for a 90-degree hybrid is satisfied), the optical hybridcircuit 1 functions as a 90-degree hybrid.

Incidentally, if the optical hybrid circuit 1 is configured in such amanner as illustrated in FIG. 3A, then where the relative phasedifference Δφ is −π/2 or +π/2, it is estimated that the characteristicmay deteriorate with output components of the third and fourth outputchannels. This arises from the fact that a phase matching is notsatisfied between the output signals of the third and fourth outputchannels of the 2:4 MMI coupler 2 and the 2:2 MMI coupler 3.

In order to prevent such deterioration of the characteristic so that90-degree hybrid operation can be obtained with certainty, it isessentially required to establish a phase matching between the outputsignals of the third and fourth output channels of the 2:4 MMI coupler 2and the 2:2 MMI coupler 3.

In particular, if the phase of light (a pair of second optical signals)outputted from one (or both) of the third and fourth output channels ofthe 2:4 MMI coupler 2 is controlled so that the relative phasedifference Δφ between a pair of second optical signals becomes π/2+p*π(p is an integer), then deterioration of the characteristic disappears.

Therefore, in the present embodiment, a phase controlling region withinwhich the phase can be controlled so that characteristic deteriorationof a quadrature phase component may not occur is provided between the2:4 MMI coupler 2 and the 2:2 MMI coupler 3. In particular, the phase oflight (a pair of second optical signals) outputted from one (or both) ofthe third and fourth output channels of the 2:4 MMI coupler 2 may becontrolled by a phase controlling region 4 so that the phase differencebetween the lights to be inputted to the two channels on the input sideof the 2:2 MMI coupler 3 may be 90 degrees. It is to be noted that thephase controlling region 4 may be configured as a region for controllingthe phase so that the phase difference between a pair of second opticalsignals may be π/2+p*π (p is an integer).

Here, as illustrated in FIG. 1A, a phase shifter 4 is provided in thephase controlling region. The phase shifter 4 is formed by varying thewidth of the optical waveguide, which connects the fourth channel on theoutput side of the 2:4 MMI coupler 2 and the second channel on the inputside of the 2:2 MMI coupler 3 to each other, in a tapered manner. Inparticular, the waveguide type phase shifter 4 which has a width varyingin a tapered manner is provided for one of the pair of output channelsof the 2:4 MMI coupler 2 to which the 2:2 MMI coupler 3 is connected.

In particular, the phase shifter 4 is formed such that, as illustratedin FIG. 8, the width of the optical waveguide between the output port ofthe 2:4 MMI coupler 2 and the input port of the 2:2 MMI coupler 3linearly increases from the output port toward a middle position in thelengthwise direction and then linearly decreases from the middleposition toward the input port. In this instance, light inputted to thesecond channel on the input side of the 2:2 MMI coupler 3 is forwardedin phase with respect to light inputted to the first channel on theinput side of the optical coupler 3.

Consequently, the optical hybrid circuit 1 outputs a pair of firstoptical signals (S−L, S+L) having an in-phase relationship with eachother and a pair of third optical signals (S−jL, S+jL) having aquadrature phase relationship with the pair of first optical signals asillustrated in FIGS. 1A and 1B, and 90-degree hybrid operation can beobtained with certainty. In short, QPSK signal light is converted into apair of first optical signals which include only an in-phase component(I-component) and a pair of third optical signals which include only aquadrature phase component (Q-component) and 90-degree hybrid operationcan be obtained with certainty by the optical hybrid circuit 1.

Here, the pair of first optical signals having an in-phase relationshipwith each other, that is, the pair of first optical signals whichinclude only an in-phase component, are a pair of optical signals whosephases are displaced by 180 degrees from each other. Meanwhile, the pairof third optical signals having a quadrature phase relationship with thepair of first optical signals, that is, the pair of third opticalsignals which include only a quadrature phase component, are a pair ofoptical signals whose phases are displaced by 90 degrees from the pairof first optical signals. It is to be noted that the pair of thirdoptical signals are a pair of optical signals whose phases are displacedby 180 degrees from each other.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 1A indicate what relative relationship the phase of LO light (L)has with reference to the phase of signal light (S). Here, S−L and S+Lindicate that they have a phase relationship displaced by 180 degreesfrom each other, and S+jL and S−jL have a phase relationship displacedby 90 degrees with respect to S+L and S−L, respectively. Further, thephase relationship diagram of FIG. 1B illustrates a phase relationshipof optical signals outputted in response to a relative phase differencebetween the QPSK signal light and the LO light.

It is to be noted that, while, in the present embodiment, the phasecontrolling region (here, the phase shifter 4) is provided in order that90-degree hybrid operation may be obtained with certainty, the provisionof the phase controlling region is not essentially required. Forexample, even if the output signals having an in-phase relationship andthe output signals having a quadrature phase relationship do not have arelationship that they have a phase difference by 90 degrees from eachother accurately and the phase thereof are displaced from each other, ifthe phase displacement is permissible to a receiving circuit includingphoto detectors, then the phase controlling region need not be provided.

Now, an example of a particular configuration of an opticalsemiconductor device which forms the optical hybrid circuit is describedwith reference to FIGS. 5 to 8.

Referring first to FIG. 5, the optical hybrid circuit 1 is formed as anoptical semiconductor device 13 which includes a GaInAsP core layer 11and an InP cladding layer 12 provided over an InP substrate 10 and has ahigh mesa waveguide structure.

Here, the 2:4 MMI coupler 2 is set in the following manner.

In particular, where the width (MMI width) of the MMI region of the 2:4MMI coupler 2 is represented by W_(M), the two input channels (inputwaveguides) are provided such that they are positioned at ⅓ and ⅔ fromthe upper side of the MMI width W_(M) in such a manner as illustrated inFIG. 6. Further, the four output channels (output waveguides) areprovided such that middle positions of the two first and second outputchannels from above and middle positions of the two third and fourthoutput channels from above are positioned at ¼ and ¾ from the upper sideof the MMI width W_(M), respectively. Furthermore, both of the distance(gap) between the two first and second output channels and the distance(gap) between the two third and fourth output channels are set equal to⅙ the MMI width W_(M).

For example, the 2:4 MMI coupler 2 is configured such that the minimumdistance between the input and output channels, that is, the distancebetween two output channels (W_(M)/6), is set to approximately 3.5 μmand the waveguide width (input/output waveguide width) W of the inputchannels and the output channels is set, for example, approximately 2.0μm so that a single mode condition is satisfied. In this instance, theMMI width W_(M) is decided to approximately 33 μm. In this instance, thelength L_(M24) of the 2:4 MMI coupler becomes approximately 758 μm.

Further, the 2:4 MMI coupler 2 is configured such that the minimumdistance between the input and output channels, that is, the distancebetween two output channels (W_(M)/6), is set to approximately 2.3 μmand the waveguide width (input/output waveguide width) W of the inputchannels and the output channels is set, for example, to approximately2.0 μm so that a single mode condition is satisfied. In this instance,the MMI width W_(M) is decided to approximately 25.8 μm. In thisinstance, the length L_(M24) of the 2:4 MMI coupler becomesapproximately 463 μm.

Meanwhile, the 2:2 MMI coupler 3 is set in the following manner.

In particular, where the 2:2 MMI coupler 3 is configured based on PI,the two input channels (input waveguides) are provided such that thecenters thereof are individually provided at W_(M)/6 from the side facesof the MMI region with reference to the MMI width W_(M) of the 2:4 MMIcoupler 2 as illustrated in FIG. 7A. Also the two output channels(output waveguides) are individually provided so as to be positioned atW_(M)/6 from the side faces of the MMI region. Further, both of thedistances (gaps) between the two input and output channels are set toW_(M)/6. Therefore, the width (MMI width) W_(M22) of the MMI region ofthe 2:2 MMI coupler 3 becomes W_(M)/2.

On the other hand, where the 2:2, MMI coupler 3 is configured based onGI, with reference to the MMI width W_(M) of the 2:4 MMI coupler 2, thetwo input channels (input waveguides) are provided such that the centersthereof are individually positioned at any other than W_(M)/6 from theside faces of the MMI region and they have a center symmetric propertyas illustrated in FIG. 7B. In other words, the two input channels areprovided such that the centers thereof are individually positioned atdistances from the side faces of the MMI region (arbitrary real numbergreater than 0 except K=W_(M)/6). Also the two output channels (outputwaveguides) are provided such that the centers thereof are individuallypositioned at any other than W_(M)/6 from the side faces of the MMIregion. In other words, the two output channels are provided such thatthe centers thereof are individually positioned at the distances K fromthe side faces of the MMI region (arbitrary real number greater than 0except K=W_(M)/6). Furthermore the distances (gaps) between the inputand output channels are individually set to W_(M)/6. Therefore, thewidth (MMI width) W_(M22) of the MMI region of the 2:2 MMI coupler 3becomes 2K+W_(M)/6.

For example, the 2:2 MMI coupler 3 based on GI is configured such thatthe minimum distance between the input/output channels, that is, thedistance between the two input channels and the distance between the twooutput channels (W_(M)/6), is set to approximately 3.5 μm and thewaveguide width (input/output waveguide width) W of the input channelsand the output channels is set, for example, approximately 2.0 μm sothat a single mode condition is satisfied. In this instance, the MMIwidth W_(M22) is decided to approximately 7.5 μm. In this instance, thelength L_(M22) of the 2:2 MMI coupler becomes approximately 235 μm.

Further, the 2:2 MMI coupler 3 based on GI is configured such that theminimum distance between the input/output channels, that is, thedistance between the two input channels and the distance between twooutput channels (W_(M)/6), is set to approximately 2.3 μm and thewaveguide width (input/output waveguide width) W of the input channelsand the output channels is set, for example, to approximately 2.0 μm sothat a single mode condition is satisfied. In this instance, the MMIwidth W_(M22) is decided to approximately 6.3 μm. In this instance, thelength L_(M22) of the 2:2 MMI coupler becomes approximately 165 μm.

Further, the phase shifter 4 is set in the following manner in order toestablish a phase matching between two signal components to be inputtedto the 2:2 MMI coupler 3.

In particular, the waveguide width W of portions of the phase shifter 4which are connected to the output ports of the 2:4 MMI coupler 2 and theinput ports of 2:2 MMI coupler 3 is set to approximately 2.0 μm asillustrated in FIG. 8. Meanwhile, the distance L_(TP) from the outputports of the 2:4 MMI coupler 2 or the input ports of the 2:2 MMI coupler3 to the middle position in the lengthwise direction are set to 100 μm.In other words, the length (taper length) of both of a width-increasingtapered portion along which the waveguide width increases linearly fromthe output ports of the 2:4 MMI coupler 2 to the middle position in thelengthwise direction and a width-decreasing tapered portion along whichthe waveguide width decreases linearly from the middle position in thelengthwise direction to the input ports of the 2:2 MMI coupler 3 are setto approximately 100 μm. In this instance, the waveguide width W_(MID)at the middle position in the lengthwise position is approximately 2.1μm. Meanwhile, the length L_(phase) of the phase shifter 4 isapproximately 200 μm.

It is to be noted that the values of the parameters regarding the phaseshifter 4, that is, the taper length L_(TP) and the waveguide widthW_(MID) at the middle position, are not limited to the values mentionedabove, but may have any value only if a phase displacement (phase shiftamount) corresponding to π/4 can be provided between two signalcomponents to be inputted to the 2:2 MMI coupler 3. For example, thetaper length L_(TP) and the waveguide width W_(MID) at the middleposition may be set to values with which a phase shift amountcorresponding, for example, to π/4×2nπ (n is an integer) can beprovided, and also in this instance, a similar effect can be achieved.Further, the taper length L_(TP) and the waveguide width W_(MID) at themiddle position may be set to approximately 20 μm and approximately 2.4μm, respectively, and also in this instance, a phase displacement quitesame as that described above can be provided. In this instance, thelength L_(phase) of the phase shifter can be set to approximately 40 μmor less and can be formed compact.

The optical hybrid circuit 1 configured as an optical semiconductordevice in this manner is fabricated in the following manner.

First, an undoped GaInAsP core layer 11 and an undoped InP claddinglayer 12 are epitaxially grown in order on an n-type InP substrate 10 asillustrated in FIG. 5, for example, by a metal organic chemical vapordeposition (MOVPE) method.

Here, the undoped GaInAsP core layer 11 has a light emission wavelengthof approximately 1.30 μm and a layer thickness of approximately 0.3 μm.Meanwhile, the undoped InP cladding layer 12 has a layer thickness ofapproximately 2.0 μm. It is to be noted that the substrate may be anundoped InP substrate. Meanwhile, the cladding layer may be a p-typedoped InP cladding layer.

Then, for example, an SiO₂ film is formed on the surface of the wafer,for which the epitaxial growth has been carried out in such a manner asdescribed above, for example, by deposition apparatus, and a waveguidepattern for forming the optical hybrid circuit 1 is patterned, forexample, by a light exposure process.

Then, using the SiO₂ film patterned in this manner as a mask, dryetching is carried out using a method such as, for example, inductivelycoupled plasma-reactive ion etching (ICP-RIE). Consequently, a high mesawaveguide stripe structure of a height of, for example, approximately 3μm is formed.

The optical hybrid circuit 1 is completed through such a fabrication asdescribed above.

Here, FIGS. 9A to 9D illustrate an input/output characteristic whereQPSK signal light (Signal) of a wavelength of 1.55 μm and LO light areinputted to the optical hybrid circuit 1 configured in such a manner asdescribed above for each relative phase difference Δφ between the QPSKsignal light and the LO light.

It is to be noted that the results of calculation illustrated in FIGS.9A to 9D are based on a beam propagation method (BPM). FIG. 9Aillustrates an input/output characteristic where the relative phasedifference Δφ is 0, FIG. 9B illustrates an input/output characteristicwhere the relative phase difference Δφ is π, FIG. 9C illustrates aninput/output characteristic where the relative phase difference Δφ is−π/2, and FIG. 9D illustrates an input/output characteristic where therelative phase difference Δφ is +π/2.

Where the relative phase difference Δφ is 0 and π as illustrated inFIGS. 9A and 9B, the output intensity ratio of the optical hybridcircuit 1 is 0:2:1:1 and 2:0:1:1, respectively.

On the other hand, where the relative phase difference Δφ is −π/2 and+π/2 as illustrated in FIGS. 9C and 9D, the output intensity ratio ofthe optical hybrid circuit 1 is 1:1:2:0 and 1:1:0:2, respectively.

In this manner, with the optical hybrid circuit 1, output forms havingdifferent branching ratios are obtained in response to the phase stateof the QPSK signal light. Further, in the optical hybrid circuit 1,since the phase shifter 4 is provided, crosstalk in a quadraturecomponent decreases significantly. Accordingly, the optical hybridcircuit 1 functions as a 90-degree hybrid with certainty.

FIG. 10A illustrates a relative output intensity (transmittance) of aconventional 90-degree hybrid [refer to FIG. 50A], which uses a 4:4 MMIcoupler, with respect to the relative phase difference Δφ, and FIG. 10Billustrates a relative output intensity (transmittance) of the opticalhybrid circuit 1 with respect to the relative phase difference Δφ.

It is to be noted that FIGS. 10A and 10B illustrate relative intensitiesof each of the output channels where the relative phase difference Δφvaries continuously.

As illustrated in FIGS. 10A and 10B, in both cases, the relative outputintensity with respect to the relative phase difference Δφ varies in asine wave function. However, in FIG. 10A, the relative output intensityis plotted reflecting the phase difference of 45 degrees whichinevitably appears in mode interference of the 4:4 MMI coupler.

As illustrated in FIGS. 10A and 10B, it is recognized that, with theoptical hybrid circuit 1, where the relative phase difference Δφ is 0,π, −π/2 and +π/2, a higher output intensity than that of theconventional 90-degree hybrid is obtained and the insertion loss is low.

Further, as illustrated in FIG. 10A, it is recognized that, with theconventional 90-degree hybrid, the output intensity variation of thefirst output channel (Ch-1) and the output intensity variation of thefourth output channel (Ch-4) have an x-axis symmetrical property.Further, it is recognized that the output intensity variation of thesecond channel (Ch-2) and the output intensity variation of the thirdoutput channel (Ch-3) have an x-axis symmetrical property.

Particularly, it is recognized that, where the relative phase differenceΔφ is 0, the output intensity of the third output channel (Ch-3) is thehighest. Further, it is recognized that, where the relative phasedifference Δφ is n, the output intensity of the second output channel(Ch-2) is the highest. Further, it is recognized that, where therelative phase difference Δφ is −π/2, the output intensity of the fourthchannel (Ch-4) is the highest. Furthermore, it is recognized that, wherethe relative phase difference Δφ is +π/2, the output intensity of thefirst output channel (Ch-1) is the highest.

In this instance, the optical signal outputted from the first outputchannel (Ch-1) and the optical signal outputted from the fourth outputchannel (Ch-4) have an in-phase relationship with each other. Meanwhile,the optical signal outputted from the second output channel (Ch-2) andthe optical signal outputted form the third output channel (Ch-3) havean in-phase relationship with each other. Further, the optical signalsoutputted from the second and third channels have a quadrature phaserelationship with the optical signals outputted from the first andfourth output channels.

This signifies that intersection of the optical waveguides cannot beavoided because optical signals outputted from the conventional90-degree hybrid are inputted to photodiodes (BPDs) for photoelectricconversion (refer to FIG. 51). Therefore, excessive loss by theintersection of optical waveguides occurs, and the light receptionefficiency deteriorates.

In contrast, as illustrated in FIG. 10B, it is recognized that, with theoptical hybrid circuit 1, the output intensity variation of the firstoutput channel (Ch-1) and the output intensity variation of the secondoutput channel (Ch-2) have an x-axis symmetrical property. Further, itis recognized that the output intensity variation of the third outputchannel (Ch-3) and the output intensity variation of the fourth outputchannel (Ch-4) have an x-axis symmetrical property.

It is recognized that, particularly where the relative phase differenceΔφ is 0, the output intensity of the second output channel (Ch-2) is thehighest. Further, it is recognized that, where the relative phasedifference Δφ is n, the output intensity of the first output channel(Ch-1) is the highest. Further, it is recognized that, where therelative phase difference Δφ is −π/2, the output intensity of the thirdoutput channel (Ch-3) is the highest. Furthermore, it is recognizedthat, where the relative phase difference Δφ is +π/2, the outputintensity of the fourth output channel (Ch-4) is the highest.

In this instance, the optical signal outputted from the first outputchannel (Ch-1) and the optical signal outputted from the second outputchannel (Ch-2) have an in-phase relationship with each other. Meanwhile,the optical signal outputted from the third output channel (Ch-3) andthe optical signal outputted form the fourth output channel (Ch-4) havean in-phase relationship with each other. Further, the optical signalsoutputted from the first and second channels have a quadrature phaserelationship with the optical signals outputted from the third andfourth output channels.

In this instance, different from the case of FIG. 4, no crosstalk occurswith output components of Ch-3 and Ch-4 which are a quadrature phasecomponent. This signifies that the relative phase difference between theoutput components of Ch-3 and Ch-4 is made proper by the phase shifter 4and a phase matching with the 2:2 MMI coupler 3 is established.

Further, it is signified that, in order to input the optical signalsoutputted from the optical hybrid circuit 1 to the photodiodes (BPDs)for photoelectric conversion, there is no necessity to lay the opticalwaveguides in an intersecting relationship as illustrated in FIG. 11.Therefore, excessive loss can be prevented.

FIG. 12A illustrates a wavelength dependency of the transmittance foreach of the four output channels where QPSK signal light is inputtedfrom one of the input channels of the conventional 90-degree hybrid[refer to FIG. 50A] which uses a 4:4 MMI coupler. Meanwhile, FIG. 12Billustrates a wavelength dependency of the transmittance in the fouroutput channels where QPSK signal light is inputted from one of theinput channels of the optical hybrid circuit 1. It is to be noted that,from whichever input channel the QPSK signal light is inputted, thecharacteristics illustrated in FIGS. 12A and 12B are substantiallysimilar to each other.

Here, in all cases, the minimum distance (gap) between the input/outputwaveguides is set to approximately 3.5 μm.

Then, if the input/output waveguide width W is approximately 2 μm, thenthe MMI width W_(M44) of the 4:4 MMI coupler, the MMI width W_(M) of the2:4 MMI coupler 2 and the MMI width W_(M22) of the 2:2 MMI coupler 3based on GI are approximately 22 μm, approximately 33 μm andapproximately 7.5 μm, respectively.

In this instance, the length L_(M44) of the 4:4 MMI coupler, the lengthL_(M24) of the 2:4 MMI coupler 2 and the length L_(M22) of the 2:2 MMIcoupler 3 are approximately 1,011 μm, approximately 758 μm andapproximately 235 μm, respectively.

Further, the length L_(TP) and the waveguide width W_(MID) at the middleposition of the phase shifter 4 provided in the optical hybrid circuit 1are approximately 20 μm and approximately 2.4 μm.

In this instance, the device length L_(Tot1) (=L_(M44)) of theconventional 90-degree hybrid which uses a 4:4 MMI coupler and thedevice length L_(Tot2) (=L_(M24)+L_(phase)+L_(M22)) of the present90-degree hybrid 1 are approximately 1,011 μm and approximately 1,033μm, respectively.

As illustrated in FIGS. 12A and 12B, the present 90-degree hybrid 1 hasa low wavelength dependency over a wavelength range of the C band incomparison with the conventional 90-degree hybrid. Further, while theconventional 90-degree hybrid exhibits a loss difference ofapproximately 5.5 dB in the maximum within the wavelength range of the Cband, the loss difference by the present 90-degree hybrid is suppressedto approximately 2.8 dB in the maximum.

FIG. 13A illustrates a wavelength dependency of the phase displacementΔψ of the conventional 90-degree hybrid [refer to FIG. 50A] which uses a4:4 MMI coupler. Meanwhile, FIG. 13B illustrates a wavelength dependencyof the phase displacement Δψ of the present 90-degree hybrid 1. It is tobe noted that the parameters of the 90-degree hybrids are similar tothose in the case of FIGS. 12A and 12B.

It is to be noted that, in FIGS. 13A and 13B, the difference (phasedisplacement amount) Δψ between an absolute phase of an output componentoutputted from each of the four output channels and a reference phase isplotted where the relative phase difference between the QPSK signallight and the LO light is 0 (Δφ=0). Here, the reference phases are aphase of an output component outputted from each of the channels in thephase relationship diagrams illustrated in FIGS. 50B and 1B. Meanwhile,the phase displacement amount is excessive phase displacement amountfrom the reference phase. Accordingly, the phase displacement amount isbetter where it is minimized. In order to demodulate a QPSK modulationsignal in error-free, it is desirable that no phase displacement occurs.Even if a phase displacement occurs, it is necessary to minimize thesame, and normally it is desirable to suppress the phase displacementamount Δψ so as to be approximately ±5 degrees or less (preferablyapproximately ±2.5 degrees or less).

As illustrated in FIGS. 13A and 13B, where it is intended to suppressthe phase displacement amount to less than ±5 degrees, the allowablebandwidth in the conventional 90-degree hybrid and the present 90-degreehybrid 1 is approximately 33 nm and approximately 38.3 nm, respectively.In particular, while the conventional 90-degree hybrid fails to coverthe entire C band range, the present 90-degree hybrid 1 can cover theentire C band range.

Further, with the present 90-degree hybrid 1, even where the phasedisplacement amount is to be suppressed to less than approximately ±2.5degrees, the bandwidth is approximately 36.4 nm and it is possible tocover almost the entire C band range.

In this manner, the present 90-degree hybrid 1 can lower the wavelengthdependency of the transmittance and the phase displacement in comparisonwith the conventional 90-degree hybrid. Such a characteristic may befurther improved by changing the parameter of the 90-degree hybrid.

As described more particularly below, the wavelength dependency of thetransmittance and the phase displacement can be further lowered, forexample, by decreasing the minimum distance (gap) between theinput/output waveguides.

Here, FIG. 14A illustrates a wavelength dependency of the transmittancefor each of the four output channels where QPSK signal light is inputtedfrom one of the input channels of the conventional 90-degree hybrid[refer to FIG. 50A] which uses a 4:4 MMI coupler. Meanwhile, FIG. 14Billustrates a wavelength dependency of the transmittance for each of thefour output channels where QPSK signal light is inputted from one of theinput channels of the present 90-degree hybrid 1. It is to be noted thatthe characteristics illustrated in FIGS. 14A and 14B are substantiallysimilar from whichever input channel the QPSK signal light is inputted.

Here, in both cases, the minimum distance (gap) between the input/outputwaveguides is set to approximately 2.3 μm.

And, where the input/output waveguide width W is set to approximately 2μm, the MMI width W_(M44) of the 4:4 MMI coupler, the MMI width W_(M) ofthe 2:4 MMI coupler 2 and the MMI width W_(M22) of the 2:2 MMI coupler 3based on GI are decided to approximately 17.2 μm, approximately 25.8 μmand approximately 6.3 μm, respectively.

In this instance, the length L_(M44) of the 4:4 MMI coupler, the lengthL_(M24) of the 2:4 MMI coupler 2 and the length L_(M22) of the 2:2 MMIcoupler 3 are approximately 620 μm, approximately 463 μm andapproximately 165 μm, respectively.

Further, the length L_(TP) and the waveguide width W_(MID) at the middleposition of the phase shifter 4 provided in the present 90-degree hybrid1 are 20 μm and 2.4 μm, respectively.

In this instance, the device length L_(Tot1) (=L_(M44)) of theconventional 90-degree hybrid which uses a 4:4 MMI coupler and thedevice length L_(Tot2) (=L_(M24)+L_(phase)+L_(M22)) of the present90-degree hybrid 1 are approximately 620 μm and approximately 668 μm,respectively.

As illustrated in FIGS. 14A and 14B, the present 90-degree hybrid 1 hasa low wavelength dependency within a wavelength range of the C band incomparison with the conventional 90-degree hybrid. Further, while, inthe conventional 90-degree hybrid, the loss difference appearing in thewavelength range of the C band is approximately 2.4 dB in the maximum,with the present 90-degree hybrid 1, the loss difference is suppressedto approximately 1.8 dB in the maximum.

FIG. 15A illustrates a wavelength dependency of the conventional90-degree hybrid [refer to FIG. 50A] which uses a 4:4 MMI coupler.Meanwhile, FIG. 15B illustrates a wavelength dependency of the phasedisplacement of the present 90-degree hybrid 1. It is to be noted thatthe parameters of the 90-degree hybrids are similar to those in the casedescribed hereinabove with reference to FIGS. 14A and 14B.

As illustrated in FIGS. 15A and 15B, where it is desired to suppress thephase displacement amount so as to be approximately ±5 degrees, both ofthe conventional 90-degree hybrid and the present 90-degree hybrid 1 cancover the entire C band range. However, the present 90-degree hybrid 1has a superior characteristic that the phase displacement amount can bekept ±1 degree or less except a region around the wavelength ofapproximately 1.53 μm.

In this manner, by decreasing the minimum distance (gap) between theinput/output waveguides, the wavelength dependency of the transmittanceand the phase displacement can be improved while the superiority overthe conventional 90-degree hybrid is maintained.

Accordingly, the optical hybrid circuit according to the presentembodiment is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and a 90-degree hybrid suitable for compactness and monolithicintegration can be implemented.

Further, since an intersecting portion of optical waveguides which isnot inevitable with the conventional 90-degree hybrid [refer to FIG.50A] which uses a 4:4 MMI coupler is not required, there is an advantagealso in that the excessive loss can be suppressed to the minimum.Furthermore, since the phase relationship of the four output signals canbe made similar to that in the conventional 90-degree hybrid [refer toFIGS. 48A and 49A], the optical hybrid circuit according to theembodiment is superior also in compatibility with 90-degree hybridswhich are currently used in coherent optical receivers, coherentdetection systems and so forth.

It is to be noted that, while, in the embodiment described above, a casewherein the 2:4 MMI coupler 2 is used as the MMI coupler at thepreceding stage is taken as an example, the MMI coupler at the precedingstage is not limited to this. The MMI coupler at the preceding stage maybe any MMI coupler which converts quadrature phase shift keying signallight into a pair of first optical signals having an in-phaserelationship with each other and a pair of second optical signals havingan in-phase relationship with each other.

For example, the 2:4 MMI coupler 2 which composes the optical hybridcircuit 1 of the embodiment described above may be replaced by a 4:4 MMIcoupler 2A having four channels on the input side thereof and havingfour channels on the output side thereof as illustrated in FIG. 16. And,if light is inputted to the two channels (a pair of input channels)provided at symmetrical positions with respect to the center position inthe widthwise direction from among the four channels on the input sideof the 4:4 MMI coupler 2A, then 90-degree hybrid operation is obtainedsimilarly as in the case of the embodiment described above.Consequently, the necessity for optical waveguides to be laid in anintersecting relationship in order to connect photo detectors as in theconventional 90-degree hybrid (refer to FIG. 51) which uses a 4:4 MMIcoupler is eliminated. It is to be noted that, in FIG. 16, like elementsto those in the embodiment described hereinabove [refer to FIG. 1A] aredenoted by like reference characters.

Here, while light is inputted to the first and fourth channels fromabove from among the four channels on the input side of the 4:4 MMIcoupler 2A, light may otherwise be inputted to the second and thirdchannels. By the configuration just described, the 4:4 MMI coupler 2Afunctions as a 180-degree hybrid similarly to the 2:4 MMI coupler 2 inthe embodiment described above.

In this instance, the 4:4 MMI coupler 2A is based on GI, and the inputchannels and the output channels can be positioned freely within a rangewithin which the center axis symmetry property of the MMI region is notlost. In particular, the positions of the first and second channels fromabove on the input side and the third and fourth channels on the inputside may be any positions only if they have a center axis symmetricproperty. Further, the positions of the first and second channels fromabove on the output side and the third and fourth channels on the outputside may be any positions only if they have a center axis symmetricproperty. However, the channel positions have some influence on thebranching characteristic.

Further, while, in the embodiment described above, a case wherein a 2:2MMI coupler is used as the 2:2 MMI coupler 3 at the succeeding stage isdescribed as an example, the optical coupler 3 is not limited to this.The optical coupler 3 at the succeeding stage may be any optical couplerwhich converts the first optical signals or the second optical signalsinto a pair of third optical signals having a quadrature phaserelationship with the first or second optical signals.

For example, the 2:2 MMI coupler 3 which composes the optical hybridcircuit 1 of the embodiment described hereinabove may be replaced by adirectional coupler (3-dB coupler; for example, a 2:2 directionalcoupler) 3A as illustrated in FIG. 17. It is to be noted that, in FIG.17, like elements to those in the embodiment described hereinabove[refer to FIG. 1A] are denoted by like reference characters. Or, the 2:2MMI coupler 3 which composes the optical hybrid circuit 1 of theembodiment described hereinabove may be replaced by a two-modeinterference coupler (for example, a 2:2 two-mode interference coupler)3B as illustrated in FIG. 18. It is to be noted that, in FIG. 18, likeelements to those in the embodiment described hereinabove [refer to FIG.1A] are denoted by like reference characters. Also in those cases,similar effects to those of the embodiment described hereinabove can beachieved. Further, although the circuits of FIGS. 17 and 18 aredescribed as modifications to the embodiment described hereinabove[refer to FIG. 1A], the modifications can be applied also to amodification wherein a 4:4 MMI coupler is used as the MMI coupler at thepreceding stage (refer to FIG. 16).

Further, while, in the embodiment described hereinabove, a case whereinthe phase shifter (phase controlling region) 4 is provided between thefourth channel (port) on the output side of the 2:4 MMI coupler 2 andthe first channel (port) on the input side of the 2:2 MMI coupler 3 istaken as an example, the provision of the phase shifter 4 is not limitedto this.

For example, the phase controlling region may be provided between thethird channel (port) on the output side of the 2:4 MMI coupler 2 and thefirst channel (port) on the input side of the 2:2 MMI coupler 3 asillustrated in FIG. 19A. In other words, a phase shifter 4A may beprovided for the other one of the pair of output channels (a pair ofsecond output channels neighboring with each other) of the 2:4 MMIcoupler 2 to which the 2:2 MMI coupler 3 is connected.

In this instance, the phase shifter 4A is similar to that in theembodiment described hereinabove in that it includes a waveguide typephase shifter wherein the width of the optical waveguides for connectingthe output ports of the 2:4 MMI coupler 2 and the input ports of the 2:2MMI coupler 3 is varied such that the optical waveguide has a taperedshape. However, the shape of the tapered optical waveguide is differentfrom that in the embodiment described hereinabove. In particular, thephase shifter 4A is formed such that the width of the optical waveguideof the output port of the 2:4 MMI coupler 2 and the input port of the2:2 MMI coupler 3 decreases linearly from the output port to the middleposition in the lengthwise direction and then increases linearly fromthe middle position toward the input port. In this instance, the phaseof light inputted to the first channel on the input side of the 2:2 MMIcoupler 3 advances with respect to light inputted to the second channelon the input side of the 2:2 MMI coupler 3.

In particular, the waveguide width W of portions of the tapered opticalwaveguide (the phase shifter 4) which are connected to the output portof the 2:4 MMI coupler 2 and the input port of the 2:2 MMI coupler 3 isapproximately 2.0 μm. Meanwhile, both of the distances L_(TP) from theoutput port of the 2:4 MMI coupler 2 and the input port of the 2:2 MMIcoupler 3 to the middle position in the lengthwise direction are set toapproximately 20 μm. In other words, the length (tepar length) of bothof a width-decreasing tapered portion along which the waveguide widthdecreases linearly from the output port of the 2:4 MMI coupler 2 to themiddle position in the lengthwise direction and a width-increasingtapered portion along which the waveguide width increases linearly fromthe middle position in the lengthwise direction to the input port of the2:2 MMI coupler 3 are set to approximately 20 μm. In this instance, thewaveguide width W_(MID) at the intermediate position in the lengthwiseposition is approximately 1.6 μm. Meanwhile, the length L_(phase) of thephase shifter is approximately 40 μm.

It is to be noted that the values of the parameters regarding the phaseshifter 4A, that is, of the taper length L_(TP) and the waveguide widthW_(MID), are not limited to the values given above, but may be set suchthat a phase displacement (phase shift amount) corresponding to π/4 maybe provided between two signal components to be inputted to the 2:2 MMIcoupler 2.

Also where the configuration described is adopted, a phase shiftcorresponding to π/4 can be provided similarly as in the embodimentdescribed hereinabove, and similar effects to those of the embodimentdescribed hereinabove can be achieved [refer to, for example, FIGS. 9and 10B].

Further, while, in the embodiment described above, a case wherein a pairof second optical signals having an in-phase relationship with eachother is converted into a pair of third optical signals having aquadrature phase relationship with the pair of first optical signals,the optical conversion is not limited to this.

For example, the optical coupler 3 may convert a pair of first opticalsignals having an in-phase relationship with each other into a pair ofthird optical signals having a quadrature phase relationship with thepair of first optical signals as illustrated in FIGS. 20A and 21A.

In this instance, the optical coupler 3 is connected to a pair of firstoutput channels neighboring with each other on the output side of theMMI coupler 2 at the preceding stage as seen in FIGS. 20A and 21A.

In particular, as illustrated in FIGS. 20A and 21A, the 2:2 MMI coupler3 is connected to the two first and second channels (that is, a pair offirst output channels neighboring with each other) from above on theoutput side of the 2:4 MMI coupler 2.

Meanwhile, the phase shifter (phase controlling region) 4 or 4A may beprovided for one of the pair of first output channels of the MMI coupler2 at the preceding stage to which the optical coupler 3 is connected asillustrated in FIGS. 20A and 21A.

For example, the phase shifter 4 may be provided between the firstchannel (port) on the output side of the 2:4 MMI coupler 2 and the firstchannel (port) on the input side of the 2:2 MMI coupler 3 as illustratedin FIG. 20A.

In this instance, as the phase shifter 4, a waveguide type phase shifterwherein the width of an optical waveguide for connecting an output portof the 2:4 MMI coupler 2 and an input port of the 2:2 MMI coupler 3 isvaried in a tapered manner may be formed in a similar manner as in thecase of the embodiment described hereinabove (refer to FIG. 8). Inparticular, the phase shifter 4 is formed such that the width of theoptical waveguide between the output port of the 2:4 MMI coupler 2 andthe input port of the 2:2 MMI coupler 3 increases linearly from theoutput port toward the middle position in the lengthwise direction andthen decreases linearly from the middle position toward the input portas illustrated in FIG. 20B. In this instance, the parameters whichdefine the structure of the phase shifter 4 may be set similarly tothose in the case of the embodiment described hereinabove (refer to FIG.8).

Or, for example, the phase shifter 4A may be provided between the secondchannel (port) on the output side of the 2:4 MMI coupler 2 and thesecond channel (port) on the input side of the 2:2 MMI coupler 3 asillustrated in FIG. 21A.

In this instance, as the phase shifter 4A, a waveguide type phaseshifter wherein the width of the optical waveguide which connects theoutput port of the 2:4 MMI coupler 2 and the input port of the 2:2 MMIcoupler 3 to each other is varied in a tapered manner may be formedsimilarly as in the case of the modification described hereinabove[refer to FIG. 19B]. In particular, the phase shifter 4A may be formedsuch that the width of the optical waveguide between the output port ofthe 2:4 MMI coupler 2 and the input port of the 2:2 MMI coupler 3decreases linearly from the output port toward the middle position inthe lengthwise direction and then increases linearly from the middleposition toward the input port as illustrated in FIG. 21B. In thisinstance, the parameters which define the structure of the phase shifter4A may be set similarly to those in the case of the modificationdescribed hereinabove [refer to FIG. 19B].

Where such a configuration as just described is adopted, the positionalrelationship of the In-phase output signals and the Quadrature outputsignals of the 90-degree hybrid is reversed from that of the embodimentand modifications described hereinabove. Further, where the relativephase difference Δφ is 0, n, −π/2 and +π/2, the output intensity ratiois 1:1:0:2, 1:1:2:0, 2:0:1:1, and 0:2:1:1, respectively.

Further, while, in the embodiment described hereinabove, a tapered phaseshifter wherein the waveguide width is varied linearly is provided inthe phase controlling region, the provision of the phase shifter is notlimited to this. For example, a tapered phase shifter wherein thewaveguide width is varied in an exponential function, a tapered phaseshifter wherein the waveguide width is varied in a sine wave function, atapered phase shifter wherein the waveguide width is varied in anelliptical function or the like may be provided. Also in those cases,similar effects can be achieved. Further, in the phase controllingregion, for example, the waveguide width may be fixed while electrodesare provided such that phase control is carried out through currentinjection or voltage application, or heater electrodes may be providedto carry out phase control through application of heat. Also in thosecases, similar effects can be achieved.

Second Embodiment

First, an optical hybrid circuit according to a second embodiment isdescribed with reference to FIGS. 22A to 30B.

The optical hybrid circuit according to the present embodiment isdifferent from that of the first embodiment described hereinabove in theconfiguration of the MMI coupler (2:4 MMI coupler) at the precedingstage and in that it includes no phase shifter. In the presentembodiment, the optical hybrid circuit 1X includes a multimodeinterference (MMI coupler) 2B at a preceding stage and an opticalcoupler 3 at a succeeding state, which are connected in cascadeconnection to each other.

Here, the MMI coupler 2B at the preceding stage is a 2:4 MMI couplerwhich has two channels on the input side thereof and four channels onthe output side thereof.

In particular, the MMI coupler 2B is a 2:4 MMI coupler which is based onpaired interference (PI). In particular, the MMI coupler 2B is a 2:4 MMIcoupler wherein the centers of the two input channels are positioned at⅓ and ⅔ from above of the width of the input end and also the positionsof the four output channels are associated with the positions of theinput channels and a higher-order modes of the (3s-1)th order (s is anatural number greater than 1) is not excited in the MMI region.Therefore, the device length can be reduced.

Here, the optical hybrid circuit 1X uses mode interference action by theMMI coupler.

Normally, interference action between modes of an MMI coupler reliesupon the reflective index, excited mode number, interference mechanismand so forth of the MMI coupler, and the amplitude relationship and thephase relationship of output signals of the MMI coupler vary dependingupon the interference action between the modes.

Here, a theory of the MMI is described briefly (refer to, for example,Lucas B. Soldano et al., “Optical Multi-Mode Interference Devices Basedon Self-Imaging: Principles and Applications”, Journal of LightwaveTechnology, Vol. 13, No. 4, pp. 615-627, April 1995, the entire contentof which is incorporated herein by reference).

FIG. 23 illustrates a schematic view of an MMI waveguide.

Usually, within an MMI waveguide (MMI region), the wave number (k_(yv))and the propagation constant (β_(v)) is associated with each other by adispersion equation given by the following expression (1):

k _(yv) ²+β_(v) ² =k ₀ ² n _(r) ²  (1)

where v is the order of the excited mode, k_(yv) is the wave number ofthe propagating transverse mode, k₀ is the wave number in the vacuum,and n_(r) is the refractive index of the MMI waveguide. It is to benoted that, if k_(yv)<<k₀n_(r) is assumed, then β_(v) can be simplifiedinto the following expression (2):

$\begin{matrix}{\beta_{v} = {{k_{0}n_{r}} - \frac{\left( {v + 1} \right)^{2}\pi \; \lambda}{4\; n_{r}W_{E}^{2}}}} & (2)\end{matrix}$

where W_(E) is an equivalent MMI width including mode leakage (alsocalled Goose-Henshen shift) into the cladding region of the MMIwaveguide. In this instance, a waveguide having a great relativerefractive index difference like a high mesa waveguide satisfies arelationship of W_(E)≈W_(M) (W_(M): physical width of the MMI waveguide;MMI width).

In this instance, the propagation constant difference between thefundamental mode and an arbitrary higher-order mode excited in the MMIwaveguide is represented by the following expression (3):

$\begin{matrix}{{{\beta_{0} - \beta_{v}} \cong \frac{{v\left( {v + 2} \right)}\pi \; \lambda}{4\; n_{r}W_{M}^{2}}} = \frac{{v\left( {v + 2} \right)}\pi}{3\; L_{\pi}}} & (3)\end{matrix}$

where L_(π) is the beat length and is a factor defined as π/(β₀−β₁).

After all, the field distribution (W) at an arbitrary position of theMMI waveguide can be represented by the following expression (4):

$\begin{matrix}{{\Psi \left( {y,z} \right)} = {\sum\limits_{v = 0}^{m - 1}\; {c_{v}{\Phi_{v}(y)}{\exp \left( {{j\left( {\beta_{0} - \beta_{v}} \right)}z} \right)}}}} & (4)\end{matrix}$

where c_(v) is the mode excitation coefficient, and Φ_(v)(y) is thetransverse mode distribution in the MMI waveguide.

As indicated by the expression (4) above, the field distribution at anarbitrary position of the MMI waveguide is represented by asuperposition of the excited modes.

The term represented by an exponential function in the expression (4)above is a term representative of a mode phase and can be represented bythe following expression

$\begin{matrix}{{\exp \left( {{j\left( {\beta_{0} - \beta_{v}} \right)}z} \right)} = {\exp \left( {j\frac{{v\left( {v + 2} \right)}\pi}{3\; L_{\pi}}z} \right)}} & (5)\end{matrix}$

In particular, the mode phase term varies depending upon the arbitraryposition z in the propagation direction of the MMI waveguide.

For example, with such a 4:4 MMI coupler as illustrated in FIG. 50A,90-degree hybrid operation is obtained.

Here, the 4:4 MMI coupler is based on general mode interference (GI:General Interference). In particular, this is a 4:4 MMI coupler whereinthe centers of the four input channels are positioned in a region exceptthe positions of ⅓, ½ and ⅔ of the MMI width W_(M) within a range withinwhich the center symmetry property of the MMI waveguide is not lost andall modes according to the MMI width W_(M) are excited.

In this instance, the minimum propagation length z^(GI) for obtaining anx equal branching characteristic (x is an integer greater than 1) isgiven by the following expression:

$\begin{matrix}{z^{GI} = \frac{3\; L_{\pi}}{P}} & (6)\end{matrix}$

Accordingly, in the case of the 4:4 MMI coupler illustrated in FIG. 50A,the minimum propagation length z^(GI) for branching into four equalbranches is 3L_(π)/4.

Meanwhile, for example, in such a 2:4 MMI coupler based on PI asillustrated in FIG. 6, the minimum propagation length z^(PI) forobtaining an x equal branching characteristic is given by the followingexpression and z^(PI) has a value equal to ⅓ that of z^(GI).

$\begin{matrix}{z^{PI} = {\frac{L_{\pi}}{P} = {\frac{1}{3}z^{GI}}}} & (7)\end{matrix}$

In particular, the expression (7) corresponds to reduction of the periodof the mode phase term in the expression (5) given hereinabove to ⅓.Where the MMI width W_(M) is equal, from the expressions (6) and (7),the 2:4 MMI coupler has an MMI length equal to ⅓ that of the 4:4 MMIcoupler.

However, as illustrated in FIGS. 50A and 6, the output channel positionsare different depending upon the interference mechanism, and even if theMMI width W_(M) is equal, the minimum distance (Gap) between theinput/output channels, that is, the distance between the outputchannels, is not equal. In any MMI coupler, it is necessary to reducethe MMI width W_(M) in order to reduce the MMI length, and also theminimum distance between the input/output channels decreasesaccordingly. It is to be noted that the minimum distance between theinput/output channels normally is a parameter restricted by afabrication technique.

Since the minimum distance between the input/output channels of the MMIcoupler based on is smaller than that of the MMI coupler based on GI asillustrated in FIGS. 50A and 6, in order to fix the minimum distancebetween the input/output channels, it is necessary to increase the MMIwidth W_(M) based on PI.

Accordingly, z^(PI) where the minimum distance between the input/outputchannels is fixed is represented by the following expression:

$\begin{matrix}{z^{PI} = {\frac{3}{4}z^{GI}}} & (8)\end{matrix}$

In particular, if the minimum distance between the input/output channelsis fixed, then the shortening effect by PI decreases to ¾ time. Anyway,since the MMI coupler based on PI always has an interaction length(propagation length: MMI length) shorter than that of the MMI couplerbased on GI, it is effective to form a compact device (light branchingand coupling device).

It is to be noted, although the MMI coupler 2B based on PI is used here,the 2:4 MMI coupler is not limited to this, but any 2:4 MMI couplerwherein a pair of input channels are provided at symmetrical positionswith respect to the center position in the widthwise direction such thatthe 2:4 MMI coupler has a center symmetrical structure may be used. Forexample, a 2:4 MMI coupler based on general interference (GI) may beused. In particular, a 2:4 MMI coupler may be used wherein the centersof the two input channels are positioned in a region except thepositions of ⅓ and ⅔ of the MMI width within a range within which thecenter symmetric property of the MMI region is not lost and all modesaccording to the MMI width are excited.

Incidentally, as described hereinabove in connection with the firstembodiment, where such a configuration as illustrated in FIG. 3A isused, it is considered that, where the relative phase difference Δφ is−π/2 and +π/2, a characteristic may deteriorate with output componentsof the third and fourth output channels.

In order to prevent deterioration of the characteristic so that90-degree hybrid operation may be obtained with certainty, it isnecessary to establish a phase matching between the output signals ofthe third and fourth output channels of the 2:4 MMI coupler 2B and the2:2 MMI coupler 3.

In particular, the deterioration of the characteristic is eliminated ifthe phase of light (a pair of second optical signals) outputted from one(or both) of the third and fourth output channels of the 2:4 MMI coupler2B is controlled so that the phase difference Δθ between the pair ofsecond optical signals becomes π/2+p*π (p is an integer).

However, since the phase difference Δθ between a pair of second opticalsignals outputted from a conventional 2:4 MMI coupler becomessubstantially equal to π/4+p*π (p is an integer), it is not easy toprevent occurrence of characteristic deterioration of a quadrature phasecomponent.

Therefore, in the present embodiment, the 2:4 MMI coupler 2B is formedsuch that it has a shape (width tapered structure) wherein the width(waveguide width) varies in a tapered manner toward the propagationdirection as illustrated in FIG. 22A so that characteristicdeterioration of a quadrature phase component may not occur. Inparticular, the width tapered structure of the 2:4 MMI coupler 2B isadopted such that the phase difference Δθ between a pair of secondoptical signals to be outputted from the 2:4 MMI coupler 2B may beπ/2+p*π (p is an integer) in order that the phase difference of thelights to be inputted to the two channels on the input side of the 2:2MMI coupler 3 may be 90 degrees.

In this instance, the 2:4 MMI coupler 2B (inclined 2:4 MMI coupler) hasan input end 2BX of a first width W_(S) and an output end 2BY of asecond width W_(M) different from the first width W_(S) and isconfigured such that the phase difference Δθ between a pair of secondoptical signals becomes π/2+p*π (p is an integer).

In particular, the 2:4 MMI coupler 2B has a tapered shape (linearfunction tapered shape) in which the width (MMI width) thereof varies ina linear function toward the propagation direction. Here, the 2:4 MMIcoupler 2B has a tapered shape in which the width thereof increaseslinearly from the input end 2BX toward the output end 2BY.

Usually, if a width taper is formed on the 2:4 MMI coupler 2B, then themode phase term in the expression (5) given hereinabove varies dependingupon the variation of the difference in propagation constant betweenexcited modes defined by the expression (3) given hereinabove, and as aresult, the field distribution represented by the expression (4) varies.Accordingly, both of the amplitude characteristic and the phasecharacteristic of the 2:4 MMI coupler 2B vary.

As illustrated in FIG. 22A, where the MMI width varies in a linearfunction, the propagation constant difference between the fundamentalmode and an arbitrary higher-order mode varies locally.

In this instance, the net phase shift (Δρ) in the MMI region isrepresented by the follow expression (9):

$\begin{matrix}{{\Delta \; \rho} = {{\int_{0}^{L_{M\; 24}}{\left( {\beta_{0} - \beta_{v}} \right)\ {z}}} = {\frac{{v\left( {v + 2} \right)}\pi \; \lambda}{4\; n_{r}}{\int_{0}^{L_{M\; 24}}\ \frac{z}{W_{M}^{2}(z)}}}}} & (9)\end{matrix}$

where W_(M)(z) represents the width taper function, and L_(M24) the 2:4MMI length.

In the case of the inclined 2:4 MMI coupler 2B illustrated in FIG. 22A,W_(M)(z) can be represented by the following expression:

$\begin{matrix}{{W_{M}(z)} = {W_{S} + {\left( {W_{M} - W_{S}} \right)\frac{z}{L_{M\; 24}}}}} & (10)\end{matrix}$

From the expressions (9) and (10) given above, the propagation constantdifference <β₀−β_(v)> between the fundamental mode and an arbitraryhigher-order mode is represented by the following expression:

$\begin{matrix}{{\langle{\beta_{0} - \beta_{v}}\rangle} = {\frac{{v\left( {v + 2} \right)}\pi \; \lambda}{4\; n_{r}W_{M}^{2}}\chi^{ST}}} & (11) \\{\chi^{ST} = \frac{W_{M}}{W_{S}}} & (12)\end{matrix}$

where χ^(ST) is a proportionality constant which relies upon the taperedshape.

From the expressions (11) and (12) given above, the beat length L_(π)^(ST) of the 2:4 MMI coupler 2B having a linear function tapered shapecan be represented by the following expression (13):

$\begin{matrix}{L_{\pi}^{ST} = \frac{L_{\pi}}{\chi^{ST}}} & (13)\end{matrix}$

Accordingly, as the proportionality constant χ^(ST) increases, the beatlength L_(π) ^(ST) of the 2:4 MMI coupler 2B having such a linearfunction tapered shape as illustrated in FIG. 22(A) decreases withrespect to the beat length L_(π) of a 2:4 MMI coupler (refer to FIG. 6)which does not have a tapered shape. It is to be noted that the MMIwidth of the 2:4 MMI coupler (refer to FIG. 6) which does not have atapered shape is set equal to the width W_(M) of the output end 2BY ofthe MMI coupler 2B having a linear function tapered shape in the presentembodiment. Here, 1/χ^(ST) is considered to be a parameter whichrepresents a shortening ratio (a reduction ratio) of the MMI lengthL_(M24) of the 2:4 MMI coupler 2B having a linear function tapered shapein the present embodiment with respect to the MMI length of the 2:4 MMIcoupler (refer to FIG. 6) which does not have a tapered shape.

Meanwhile, the phase shift in the 2:4 MMI coupler 2B relies upon thetapered shape which varies like a linear function.

Therefore, by setting χ^(ST) so that, where Δφ is π/2 and +π/2, thephase difference Δθ between a pair of second optical signals to beoutputted from the 2:4 MMI coupler 2B may become π/2+p*π (p is aninteger), 90-degree hybrid operation is obtained with certainty. Inother words, by setting χ^(ST) to a proper value, such an optical hybridcircuit as illustrated in FIG. 22A functions as a 90-degree hybrid. Alsoshortening of the 2:4 MMI length L_(M24) can be anticipated.

Here, FIG. 24 illustrates a relationship between the variation ratio ofthe MMI width of the 2:4 MMI coupler 2B, that is, the ratio W_(M)/W_(S)between the width W_(S) of the input end 2BX of the 2:4 MMI coupler 2Band the width W_(M) of the output end 2BY of the 2:4 MMI coupler 2B andthe absolute value |Δθ| of the inter-channel phase difference of theoutput signals. It is to be noted that, in FIG. 24, the absolute value|Δθ| of the inter-channel phase difference of the output signals is anabsolute value of the inter-channel phase difference between the outputsignals (a pair of second optical signal) outputted from the third andfourth output channels of the 2:4 MMI coupler 2B.

Meanwhile, FIG. 25 illustrates a relationship between the variationratio W_(M)/W_(S) of the MMI width of the 2:4 MMI coupler 2B and thereduction ratio of the 2:4 MMI length L_(M24), that is, 1/χ^(ST) (thatis, W_(S)/W_(M)).

It is to be noted that, where the value of W_(M)/W_(S) is 1, the 2:4 MMIcoupler has no tapered shape (refer to FIG. 6). Here, the width W_(M) ofthe output end 2BY of the 2:4 MMI coupler 2B is made equal (fixed) withrespect to the 2:4 MMI coupler which has no tapered shape while thewidth W_(S) of the input end 2BX is varied. It is to be noted that thewidth W_(S) of the input end 2BX of the MMI coupler 2B may be fixedwhile the width W_(M) of the output end 2BY is varied.

As illustrated in FIGS. 24 and 25, as the value of W_(M)/W_(S)increases, the value of the absolute value |Δθ| of the inter-channelphase difference between the output signals increases linearly and thevalue of 1/χ^(ST) decreases (the MMI length L_(M24) is shortened).

Therefore, by setting the value of the width W_(S) of the input end 2BXof the 2:4 MMI coupler 2B (that is, the value of W_(M)/W_(S)), theabsolute value |Δθ| of the inter-channel phase difference between theoutput signals can be set to the desired value of π/2.

Here, as illustrated in FIG. 24, where the absolute value |Δθ| of theinter-channel phase difference between the output signals the value ofW_(M)/W_(S) is π/2 is 2, and as illustrated in FIG. 25, where the valueof W_(M)/W_(S) is 2 the value of 1/χ^(ST) is approximately 0.48. In thisinstance, the value of χ^(ST) is approximately 2.06.

Accordingly, the MMI length L_(M24) of the 2:4 MMI coupler 2B having alinear function tapered shape is 1/χ^(ST) time, that is, approximately0.48 times, the MMI length of the 2:4 MMI coupler (refer to FIG. 6)which does not have a tapered shape. In other words, the MMI lengthL_(M24) of the MMI coupler 2B which has a linear function taper is equalto or smaller than one-half the MMI length of the 2:4 MMI coupler (referto FIG. 6) which does not have a tapered shape.

Further, the relationships illustrated in FIGS. 24 and 25 are satisfiedwith regard to an arbitrary width W_(S) of the input end 2BX and anarbitrary width W_(M) of the output end 2BY. In other words, if anarbitrary width W_(S) of the input end 2BX and an arbitrary width W_(M)of the output end 2BY satisfy the condition of X^(ST)=approximately2.06, then |Δθ|=π/2 is satisfied, and 90-degree hybrid operation isobtained with certainty using the 2:4 MMI coupler 2B having such alinear function tapered shape as illustrated in FIG. 22A.

In particular, where the width W_(M) of the output end 2BY of the 2:4MMI coupler 2B is approximately 33 μm, the width W_(S) of the input end2BX is decided to approximately 16 μm (refer to FIG. 24) from thecondition that the absolute value |Δθ| of the inter-channel phasedifference of the output signals is (|Δθ|=π/2). If the width W_(M) ofthe output end 2BY and the width W_(S) of the input end 2BX of the 2:4MMI coupler 2B are decided in this manner, then the length L_(M24) ofthe 2:4 MMI coupler 2B is decided to approximately 368 μm from thecondition of 1/χ^(ST)=approximately 0.48, that is approximately 2.06.The linear function tapered shape of the 2:4 MMI coupler 2B is definedby an expression [taper function W_(M)(z)] obtained by substituting thevalues given above into the expression (10) given hereinabove.

If the width W_(M) of the output end 2BY of the MMI coupler 2B having alinear function tapered shape is set to approximately 26.4 μm(corresponding to approximately 80% of the approximately 33 μm), thenthe desired width W_(S) of the input end 2BX becomes approximately 12.8μm (corresponding to approximately 80% of approximately 16 μm).

Since a linear function tapered shape is defined in such a manner asdescribed above and the MMI width is modulated with a predeterminedtaper function, the phase difference Δθ between a pair of second opticalsignals outputted from the 2:4 MMI coupler 2B becomes π/2+p*π (p is aninteger). Therefore, the optical hybrid circuit 1×outputs a pair offirst optical signals (S−L and S+L) having an in-phase relationship witheach other and a pair of third optical signals (S−jL and S+jL) having aquadrature phase relationship with the pair of first optical signals asillustrated in FIGS. 22A and 22B. Consequently, 90-degree hybridoperation is obtained with certainty. In short, QPSK signal light isconverted into a pair of first optical signals which include only anin-phase component (I-component) and a pair of third optical signalswhich include only a quadrature phase components (Q-component) asillustrated in FIGS. 22A and 22B by the optical hybrid circuit 1X, andconsequently, 90-degree hybrid operation is obtained with certainty.

Here, the pair of first optical signals having an in-phase relationshipwith each other, that is, the pair of first optical signals whichinclude only an in-phase component, are a pair of optical signals whosephases are displaced by 180 degrees from each other. On the other hand,the pair of third optical signals having a quadrature phase relationshipwith the pair of first optical signals, that is, the pair of thirdoptical signals which include only a quadrature phase component, are apair of optical signals whose phases are displaced by 90 degrees fromthose of the pair of first optical signals. It is to be noted that thepair of third optical signals are a pair of optical signals whose phasesare displaced by 180 degrees from each other.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 22A indicate what relative relationship the phase of LO light (L)has with reference to the phase of signal light (S). Here, S−L and S+Lindicate that they have a phase relationship displaced by 180 degreesfrom each other, and S+jL and S−jL have a phase relationship displacedby 90 degrees with respect to S+L and S−L, respectively. Further, thephase relationship diagram of FIG. 22B illustrates a phase relationshipof optical signals outputted in response to a relative phase differencebetween the QPSK signal light and the LO light.

Now, examples (refer to FIGS. 5, 7A and 7B) of a particularconfiguration of the optical semiconductor device which forms theoptical hybrid circuit of the present embodiment are described.

The present optical hybrid circuit 1X is an optical semiconductor device13 which includes a GaInAsP core layer 11 and an InP cladding layer 12provided on an InP substrate 10 and has a high mesa waveguide structuresimilarly as in the case of the first embodiment described hereinabove.

Here, the 2:4 MMI coupler 2B is set in the following manner.

In particular, where the width of the input end 2BX of the 2:4 MMIcoupler 2B is W_(S), the two input channels (input waveguides) areprovided such that the centers thereof are positioned at ⅓ and ⅔ fromabove of the width W_(S) of the input end 2BX of the 2:4 MMI coupler 2B.Further, where the width of the output end 2BY of the 2:4 MMI coupler 2Bis W_(M), the four output channels (output waveguides) are provided suchthat a middle position of the two first and second output channels fromabove and a middle position of the two third and fourth output channelsfrom above are positioned at ¼ and ¾ from above of the width W_(M) ofthe output end 2BY of the 2:4 MMI coupler 2B, respectively. Further, thedistance (gap) between the two first and second output channels and thedistance (gap) between the two third and fourth output channels are bothset to ⅙ the width W_(M) of the output end 2BY of the 2:4 MMI coupler2B. It is to be noted here that the width W_(M) of the output end 2BY ofthe 2:4 MMI coupler 2B is equal to the MMI width of the 2:4 MMI coupler(refer to FIG. 6) which does not have a tapered shape.

For example, the 2:4 MMI coupler 2B is configured such that the minimumdistance between the input/output channels, that is, the distance(W_(M)/6) between two output channels, is set to approximately 3.5 μmand the waveguide width (input/output waveguide width) W of the inputchannels and the output channels is set, for example, to approximately2.0 μm such that the single mode condition is satisfied. Consequently,the width W_(M) of the output end 2BY of the 2:4 MMI coupler 2B isdecided to approximately 33 μm. Further, in order to establish a phasematching between the two signal components to be inputted to the 2:2 MMIcoupler 3, the width W_(S) of the input end 2BX of the 2:4 MMI coupler2B is decided to approximately 16 μm from the condition of the absolutevalue |Δθ| of the inter-channel phase difference of the output signalsis π2 (|Δθ|=π/2). If the width W_(S) of the input end 2BX and the widthW_(M) of the output end 2BY of the 2:4 MMI coupler 2B is decided in thismanner, then the length L_(M24) of the MMI coupler 2B is decided to 368μm from the condition of 1/χ^(ST)=approximately 0.48, that is,χ^(ST)=approximately 2.06. As a result, the MMI width of the 2:4 MMIcoupler 2B comes to have a tapered shape (linear function tapered shape)wherein the width increases linearly from the input end 2BX toward theoutput end 2BY.

It is to be noted that the parameters regarding the 2:4 MMI coupler 2Bare not limited to the values given above, but only it is necessary toset the parameters so that a phase displacement (phase shift amount)corresponding to π/4 can be provided between the two signal componentsto be inputted to the 2:2 MMI coupler 3. For example, only it isnecessary to provide a phase shift amount corresponding to π/4×2nπ (n isan integer). Alto in this instance, similar effects are achieved.

Further, the 2:2 MMI coupler 3 is set in the following manner.

In particular, where the 2:2 MMI coupler 3 is based on PI [refer to FIG.7A], the two input channels (input waveguides) are provided such thatthe centers thereof are positioned at W_(M)/6 from the side faces of theMMI region with reference to the width W_(M) of the output end 2BY ofthe 2:4 MMI coupler 2B. Also the two output channels (output waveguides)are provided such that the centers thereof are positioned at W_(M)/6from the side faces of the MMI region. Furthermore, both of thedistances (gaps) between the two input/output channels are set toW_(M)/6. Therefore, the width (MMI width) W_(M22) of the MMI region ofthe 2:2 MMI coupler 3 becomes W_(M)/2.

Meanwhile, where 2:2 MMI coupler 3 is based on GI [refer to FIG. 7(B)],the two input channels (input waveguides) are provided such that thecenters thereof are positioned at any other than W_(M)/6 from the sidefaces of the MMI region and have a center symmetrical property withreference to the width W_(M) of the output end 2BY of the 2:4 MMIcoupler 2B. In other words, the two input channels are individuallyprovided such that the centers thereof are positioned at distances K(arbitrary real number greater than 0 except K=W_(M)/6) from the sidefaces of the MMI region. Also the two output channels (outputwaveguides) are provided such that the centers thereof are individuallypositioned at any other than W_(M)/6 from the side faces of the MMIregion. In other words, the two output channels are provided such thatthe centers thereof are individually positioned at distances K(arbitrary real number greater than 0 except K=W_(M)/6) from the sidefaces of the MMI region. Furthermore, both of the distances (gaps)between the two input/output channels are set to W_(M)/6. Therefore, thewidth (MMI width) W_(M22) of the MMI region of the 2:2 MMI coupler 3becomes 2K+W_(M)/6.

For example, the 2:2 MMI coupler 3 based on GI is configured such thatthe minimum distance between the input/output channels, that is, thedistance (W_(M)/6) between the two input channels and between the twooutput channels, is set to approximately 3.5 μm and the waveguide width(input/output waveguide width) W of the input channels and the outputchannels is set, for example, to approximately 2.0 μm such that thesingle mode condition is satisfied. Consequently, the MMI width W_(M22)is decided to approximately 7.5 μm. In this instance, the length L_(M22)of the 2:2 MMI coupler becomes approximately 235 μm.

Here, FIGS. 26A to 26D illustrate an input/output characteristic whereQPSK signal light (Signal) and LO light of a wavelength of approximately1.55 μm are inputted to the optical hybrid circuit 1X configured in sucha manner as described above for each of the relative phase differencesΔφ between the QPSK signal light and the LO light. It is to be notedthat the 2:4 MMI coupler 2B is configured such that the width W_(M) ofthe output end 2BY is approximately 33 μm and the width W_(S) of theinput end 2BX is approximately 16 μm and consequently the 2:4 MMIcoupler 2B has a linear function tapered shape which satisfies thecondition of χ^(ST)=approximately 2.06.

It is to be noted that the calculation results illustrated in FIGS.26(A) to 26(D) are based on a beam propagation method (BPM). Inparticular, FIG. 26(A) illustrates an input/output characteristic wherethe relative phase difference Δφ is 0; FIG. 26B illustrates aninput/output characteristic where the relative phase difference Δφ is π;FIG. 26C illustrates an input/output characteristic where the relativephase difference Δφ is −π/2; and FIG. 26D illustrates an input/outputcharacteristic where the relative phase difference Δφ is +π/2.

As illustrated in FIGS. 26A and 26B, where the relative phase differenceΔφ is 0 and π, the output intensity ratios of the optical hybrid circuit1X are 0:2:1:1 and 2:0:1:1, respectively.

On the other hand, as illustrated in FIGS. 26(C) and 26D, where therelative phase difference Δφ is π/2 and +π/2, the output intensityratios of the optical hybrid circuit 1X are 1:1:2:0 and 1:1:0:2,respectively.

In this manner, with the present optical hybrid circuit 1X, output formshaving different branching ratios from each other are obtained inresponse to different phase states of QPSK signal light.

Further, the present optical hybrid circuit 1X uses a 2:4 MMI coupler 2Bhaving a linear function tapered shape wherein the width W_(M) of theoutput end 2BY and the width W_(S) of the input end 2BX of the 2:4 MMIcoupler 2B satisfy the condition of |Δθ|=π/2 and the length L_(M24) ofthe MMI coupler 2B satisfies the condition of χ^(ST)=approximately 2.06.Therefore, no crosstalk occurs between the output signals from the 2:2MMI coupler 3. Accordingly, the optical hybrid circuit 1X functions as a90-degree hybrid.

FIG. 27A illustrates a relative output intensity (transmittance) withrespect to the relative phase difference Δφ of the conventional90-degree hybrid [refer to FIG. 50A] which uses a 4:4 MMI coupler, andFIG. 27B illustrates a relative output intensity (transmittance) withrespect to the relative phase difference Δφ of the present 90-degreehybrid.

It is to be noted that FIGS. 27A and 27B illustrate relative intensityof each of the output channels where the relative phase difference Δφvaries continuously.

As illustrated in FIGS. 27A and 27B, in all cases, the relative outputintensity with respect to the relative phase difference Δφ varies in asine wave function. However, in FIG. 27A, the relative output intensityis plotted reflecting the phase difference of 45 degrees which isinevitably caused by mode interference of the 4:4 MMI coupler.

As illustrated in FIG. 27A, it can be recognized that, with theconventional 90-degree hybrid, the output intensity variation of thefirst output channel (Ch-1) and the output intensity variation of thefourth output channel (Ch-4) have a relationship in phase displaced by180 degrees from each other. Further, it can be recognized that theoutput intensity variation of the second output channel (Ch-2) and theoutput intensity variation of the third output channel (Ch-3) have arelationship in phase displaced by 180 degrees from each other.

It can be recognized that, particularly where the relative phasedifference Δφ is 0, the output intensity of the third output channel(Ch-3) is the highest. Meanwhile, it is recognized that, where therelative phase difference Δφ is n, the output intensity of the secondoutput channel (Ch-2) is the highest. Further, it is recognized that,where the relative phase difference Δφ is −π/2, the output intensity ofthe fourth output channel (Ch-4) is the highest. Further, it isrecognized that, where the relative phase difference Δφ is +π/2, theoutput intensity of the first output channel (Ch-1) is the highest.

In this instance, the optical signal outputted from the first outputchannel (Ch-1) and the optical signal outputted from the fourth outputchannel (Ch-4) have an in-phase relationship with each other. Meanwhile,the optical signal outputted from the second output channel (Ch-2) andthe optical signal outputted from the third output channel (Ch-3) havean in-phase relationship with each other. Further, the optical signalsoutputted from the second and third output channels have a quadraturephase relationship with the optical signals outputted from the first andfourth output channels.

This signifies that, in order to input optical signals outputted fromthe conventional 90-degree hybrid to photodiodes (BPDs) forphotoelectric conversion, intersection of optical waveguides cannot beavoided (refer to FIG. 51). Therefore, excessive loss is caused by theintersection of the optical waveguides, and the optical receptionefficiency deteriorates.

In contrast, as illustrated in FIG. 27B, it is recognized that, in thepresent 90-degree hybrid 1, the phases of the output intensity variationof the first output channel (Ch-1) and the output intensity variation ofthe second output channel (Ch-2) have a relationship that they aredisplaced by 180 degrees from each other. Further, it is recognized thatthe phases of the output intensity variation of the third output channel(Ch-3) and the output intensity variation of the fourth output channel(Ch-4) have a relationship that they are displaced by 180 degrees fromeach other.

It can be recognized that, particularly where the relative phasedifference Δφ is 0, the output intensity of the second output channel(Ch-2) is the highest. Meanwhile, it is recognized that, where therelative phase difference Δφ is π, the output intensity of the firstoutput channel (Ch-1) is the highest. Further, it is recognized that,where the relative phase difference Δφ is −π/2, the output intensity ofthe third output channel (Ch-3) is the highest. Further, it isrecognized that, where the relative phase difference Δφ is +π/2, theoutput intensity of the fourth output channel (Ch-4) is the highest.

In this instance, the optical signal outputted from the first outputchannel (Ch-1) and the optical signal outputted from the second outputchannel (Ch-2) have an in-phase relationship with each other. Meanwhile,the optical signal outputted from the third output channel (Ch-3) andthe optical signal outputted from the fourth output channel (Ch-4) havean in-phase relationship with each other. Further, the optical signalsoutputted from the third and fourth output channels have a quadraturephase relationship with the optical signals outputted from the first andsecond output channels.

In this instance, different from the example illustrated in FIG. 4, nocrosstalk occurs with the output components of third output channel(Ch-3) and the fourth output channel (Ch-4) which are quadrature phasecomponents. This signifies that the relative phase difference betweenthe output components of Ch-3 and Ch-4 are made proper by the 2:4 MMIcoupler 2B having a linear function tapered shape and a phase matchingwith the 2:2 MMI coupler 3 is established.

Further, it is signified that it is not necessary to make the opticalwaveguides intersect with each other as illustrated in FIG. 28 in orderto input the optical signals outputted from the present 90-degree hybrid1 to the photodiodes (BPDs) for photoelectric conversion. Therefore,excessive loss can be prevented.

FIG. 29A illustrates a wavelength dependency of the transmittance foreach of the four output channels where QPSK signal light is inputtedfrom one of the input channels of the conventional 90-degree hybrid[refer to FIG. 50A] which uses a 4:4 MMI coupler. Meanwhile, FIG. 29Billustrates a wavelength dependency of the transmittance for each of thefour output channels where QPSK signal light is inputted from one of thechannels of the optical hybrid circuit 1. It is to be noted that thecharacteristics illustrated in FIGS. 29A and 29B are similar fromwhichever one of the input channels QPSK signal light is inputted.

Here, in all cases, the minimum distance (gap) between the input/outputwaveguides is set to approximately 3.5 μm.

Then, if the input/output waveguide width W is approximately 2 μm, thenthe MMI width W_(M44) of the 4:4 MMI coupler, the width W_(M) of theoutput end 2BY and the width W_(S) of the input end 2BX of the MMIcoupler 2B and the MMI width W_(M22) of the 2:2 MMI coupler 3 based onGI are approximately 22 μm, approximately 33 μm, approximately 16 μm andapproximately 7.5 μm, respectively.

In this instance, the length L_(M44) of the 4:4 MMI coupler, the lengthL_(M24) of the 2:4 MMI coupler 2B and the length L_(M22) of the 2:2 MMIcoupler 3 are approximately 1,011 μm, approximately 368 μm andapproximately 235 μm.

It is to be noted that, in the present 90-degree hybrid 1, the length(an access wavelength length) L_(A) of an optical waveguide 4B (anaccess waveguide; an access region) for connecting the 2:4 MMI coupler2B and the 2:2 MMI coupler 3 to each other is approximately 20 μm (referto FIG. 22A).

In this instance, the device length L_(Tot1) (=L_(M44)) of theconventional 90-degree hybrid which uses a 4:4 MMI coupler and thedevice length L_(Tot2) (=L_(M24)+L_(A)+L_(M22)) of the 90-degree hybrid1 are approximately 1,011 μm and approximately 622 μm, respectively.Accordingly, with the 90-degree hybrid 1, the device length can beshortened by approximately 40% in comparison with the conventional90-degree hybrid which uses a 4:4 MMI coupler.

As illustrated in FIGS. 29A and 29B, the present 90-degree hybrid 1 hasa significantly reduced wavelength dependency of the transmittancewithin the wavelength range of the C band in comparison with theconventional 90-degree hybrid. On the other hand, while, with theconventional 90-degree hybrid, the loss difference which occurs in thewavelength range of the C band is approximately 5.5 dB in the maximum,with the present 90-degree hybrid, the loss difference is suppressed toapproximately 1.1 dB in the maximum.

FIG. 30A illustrates a wavelength dependency of the phase displacementin the conventional 90-degree hybrid [refer to FIG. 50A] which uses a4:4 MMI coupler. Meanwhile, FIG. 30B illustrates a wavelength dependencyof the phase displacement grin the present 90-degree hybrid 1. It is tobe noted that the parameters of the 90-degree hybrid are similar tothose in the cases described hereinabove with reference to FIGS. 29A and29B.

It is to be noted that, in FIGS. 30A and 30B, the difference (phasedisplacement amount) Δψ between the absolute phase of an outputcomponent outputted from each of the four output channels and areference phase where the relative phase difference between QPSK signallight and LO light is 0 (Δφ=0) is plotted. Here, the reference phase isa phase of an output component outputted from each of the channels inthe phase relationship diagrams of FIGS. 50B and 22B. Meanwhile, thephase displacement amount is an excessive phase displacement amount fromthe reference phase. Accordingly, the smaller the phase displacementamount, the better. In order to demodulate a QPSK modulation signal inerror-free, it is desirable that no phase displacement occurs. Even ifphase displacement occurs, it is preferable to suppress it to theminimum, and usually it is desirable to suppress the phase displacementamount Δψ so as to be ±5 degrees or less (preferably ±3 degrees orless).

As illustrated in FIGS. 30A and 30B, where it is intended to suppressthe phase displacement amount Δψ so as to be ±5 degrees or less, thepermissible bandwidths of the conventional 90-degree hybrid and thepresent 90-degree hybrid 1 are approximately 33 nm or more andapproximately 40 nm or more, respectively. In particular, while, withthe conventional 90-degree hybrid, the entire C band range cannot becovered, with the present 90-degree hybrid 1, the entire C band rangecan be covered.

Further, with the present 90-degree hybrid 1, the C band range (region)can be covered where the phase displacement Δψ is within a range ofapproximately ±3 degrees.

It is to be noted that, since details of the other part are similar tothose of the first embodiment described hereinabove, overlappingdescription of them is omitted herein.

Accordingly, the optical hybrid circuit according to the presentembodiment is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and a 90-degree hybrid suitable for compactness and monolithicintegration can be implemented.

Further, since an intersecting portion of optical waveguides whichcannot be avoided with the conventional 90-degree hybrid [refer to FIG.50A] which uses a 4:4 MMI coupler is not required, there is an advantagealso in that excessive loss can be suppressed to the minimum. Further,since the phase relationship of the four output signals can be madesimilar to that of the conventional 90-degree hybrid [refer to FIGS. 48Aand 49A], the 90-degree hybrid according to the present embodiment issuperior also in compatibility with 90-degree hybrids which are used incoherent optical receivers, coherent detection systems and so forth atpresent.

It is to be noted that, while the description of the embodimentdescribed above is given taking a case wherein the 2:4 MMI coupler 2B isused as the MMI coupler at the preceding stage as an example, the MMIcoupler at the preceding stage is not limited to this. The MMI couplerat the preceding stage may be any MMI coupler which converts quadraticphase shift keying modulation signal light into a pair of first opticalsignals having an in-phase relationship with each other and a pair ofsecond optical signals having an in-phase relationship with each other.

For example, the 2:4 MMI coupler 2B which composes the optical hybridcircuit 1X of the embodiment described may be replaced by the 4:4 MMIcoupler 2A which has four channels on the input side and four channelson the output side thereof. Thus, if light is inputted to two channels(a pair of input channels) which are provided at symmetrical positionswith respect to the center position in the widthwise direction fromamong the four channels on the input side of the 4:4 MMI coupler 2A,then 90-degree hybrid operation is obtained similarly as in theembodiment described above. Consequently, the necessity to make opticalwaveguides intersect with each other in order to connect them to photodetectors as in the conventional 90-degree hybrid (refer to FIG. 51)which uses a 4:4 MMI coupler is eliminated.

Here, while light is inputted to the first channel and the fourthchannel from above from among the four channels on the input side of the4:4 MMI coupler 2A, light may otherwise be inputted to the secondchannel and the third channel. According to this input scheme, the 4:4MMI coupler 2A functions as a 180-degree hybrid similarly to the 2:4 MMIcoupler 2B of the embodiment described above.

In this instance, the 4:4 MMI coupler 2A is based on GI and the inputchannels and the output channels can be freely positioned within a rangewithin which the center axis symmetrical property of the MMI region isnot lost. In particular, the first and second channels from above on theinput side and the third and fourth input channels from above on theinput side may be positioned at any positions only if the center axissymmetrical property is maintained. Further, the first and secondchannels from above on the output side and the third and fourth channelsfrom above on the output side may be positioned at any positions only ifthe center axis symmetrical property is maintained. However, the channelpositions have some influence on the branching characteristic.

Further, while, in the description of the embodiment described above, acase wherein a 2:2 MMI coupler is used as the optical coupler 3 at thesucceeding state is taken as an example, the optical coupler 3 is notlimited to this. The optical coupler 3 at the succeeding stage may beany optical coupler only if it converts the first optical signals or thesecond optical signals into a pair of third optical signals having aquadrature phase relationship with the first or second optical signals.

Further, while, in the embodiment described above, description is giventaking a case wherein the 2:2 MMI coupler 3 is connected to the twothird and fourth channels (that is, a pair of second output channelsneighboring with each other) from above on the output side of theinclined 2:4 MMI coupler 2B such that the optical coupler 3 converts apair of second optical signals having an in-phase relationship with eachother into a pair of third optical signals having a quadrature phaserelationship with the pair of first optical signals as an example, theconversion by the 2:2 MMI coupler 3 is not limited to this.

For example, a pair of first optical signals having an in-phaserelationship with each other may be converted into a pair of thirdoptical signals having a quadrature phase relationship with the pair ofsecond optical signals as illustrated in FIG. 31.

In this instance, the optical coupler 3 is connected to a pair of firstoutput channels, which neighbor with each other, on the output side ofthe MMI coupler 2 at the preceding stage as illustrated in FIG. 31.

In particular, as illustrated in FIG. 31, the 2:2 MMI coupler 3 isconnected to the two first and second channels from above on the outputside of the inclined 2:4 MMI coupler 2B (in particular, to a pair offirst output channels neighboring with each other).

Where such a configuration as just described is adopted, the positionalrelationship of the In-phase output signals and the Quadrature outputsignals of the 90-degree hybrid is reversed from that of the embodimentand modifications described hereinabove. Further, where the relativephase difference Δφ is 0, n, −π/2 and +π/2, the output intensity ratiois 1:1:2:0, 1:1:0:2, 2:0:1:1 and 0:2:1:1, respectively.

Further, while, in the embodiment described above, the 2:4 MMI coupler2B and the 2:2 MMI coupler 3 are connected to each other by the opticalwaveguide (access waveguide) 4B, the connection scheme between them isnot limited to this. For example, as illustrated in FIG. 32, dependingupon the length L_(M24) of the 2:4 MMI coupler 2B, the 2:4 MMI coupler2B and the 2:2 MMI coupler 3 may be connected directly to each otherwithout the intervention of the access waveguide 4B. Also in thisinstance, similar action and effects can be achieved.

FIGS. 33A to 33D illustrate an input/output characteristic where QPSKsignal light and LO light are inputted to the optical hybrid circuit 1Xhaving such a configuration as just described for each relative phasedifference Δφ between the QPSK signal light and the LO light. It is tobe noted that the device parameters are similar to those in theembodiment described hereinabove (refer to FIG. 22A).

It is to be noted that the calculation results illustrated in FIGS. 33Ato 33D are based on a beam propagation method (BPM). FIG. 33Aillustrates an input/output characteristic where the relative phasedifference Δφ is 0; FIG. 33B illustrates an input/output characteristicwhere the relative phase difference Δφ is π; FIG. 33C illustrates aninput/output characteristic where the relative phase difference Δφ is−π/2; and FIG. 33D illustrates an input/output characteristic where therelative phase difference Δφ is +π/2.

As illustrated in FIGS. 33A and 33B, where the relative phase differenceΔφ is 0 and n, the intensity ratio of the optical hybrid circuit 1X is0:2:1:1 and 2:0:1:1, respectively.

Meanwhile, as illustrated in FIGS. 33(C) and 33(D), where the relativephase difference Δφ is −π/2 and +π/2, the output intensity ratio of theoptical hybrid circuit 1X is 1:1:2:0 and 1:1:0:2, respectively.

In this manner, even where the 2:4 MMI coupler 2B and the 2:2 MMIcoupler 3 are connected directly to each other, output forms havingdifferent branching ratios from each other are obtained with respect tothe phase state of the QPSK signal similarly as in the case of theembodiment described above. Further, in the present optical hybridcircuit 1X, the 2:4 MMI coupler is used which has a linear functiontapered shape with which the width W_(M) of the output end 2BY and thewidth W_(S) of the input end 2BX of the 2:4 MMI coupler 2B satisfy thecondition of |Δθ|=π/2 and the length L_(M24) of the 2:4 MMI coupler 2Bsatisfies the condition of χ^(ST)=approximately 2.06. Therefore,crosstalk does not occur between the output signals from the 2:2 MMIcoupler 3. Accordingly, the present optical hybrid circuit 1X functionsas a 90-degree hybrid. Further, where the 2:4 MMI coupler 2B and the 2:2MMI coupler 3 are connected directly to each other, further shorteningof the device length and simplification of the structure can beachieved.

Further, for example, the 2:2 MMI coupler 3 which forms the opticalhybrid circuit 1X of the embodiment described above may be replaced by adirectional coupler (3-dB coupler; for example, a 2:2 directionalcoupler) 3A. It is to be noted that, in FIG. 34, like elements to thoseof the embodiment described hereinabove [refer to FIG. 22A] are denotedby like reference characters. Further, for example, the 2:2 MMI coupler3 which composes the optical hybrid circuit 1X of the embodimentdescribed above may be replaced by a two-mode interference coupler 3B(for example, a 2:2 two-mode interference coupler) as illustrated inFIG. 35. It is to be noted that, in FIG. 35, like elements to those ofthe embodiment described above [refer to FIG. 22A] are denoted by likereference characters. Also in those cases, similar effects to those ofthe embodiment described hereinabove are obtained. Further, while thedirectional coupler 3A and the two-mode interference coupler 3B aredescribed as modifications to the embodiment described hereinabove[refer to FIG. 22A], also it is possible to apply the modifications to amodification wherein a 4:4 MMI coupler is used as the MMI coupler at thepreceding stage.

It is to be noted that, while the foregoing description of theembodiment described above is given taking a case wherein InP is used asa semiconductor material as an example, the semiconductor material isnot limited to this. For example, some other group III-V compoundsemiconductor material (for example, GaAS), a group IV semiconductormaterial (for example, Si) or the like may be used to form a similarwaveguide structure. Also in this instance, similar action and effectscan be achieved.

Third Embodiment

Now, an optical hybrid circuit according to a third embodiment isdescribed with reference to FIGS. 36 to 38.

The optical hybrid circuit according to the present embodiment isdifferent in the tapered shape of the 2:4 MMI coupler from that of thesecond embodiment described hereinabove.

In particular, in the present optical hybrid circuit, the 2:4 MMIcoupler 2C has a tapered shape (square function tapered shape; widthtapered structure) wherein the width (MMI width; waveguide width)thereof varies in a square function toward the propagation direction asillustrated in FIG. 36. Here, the 2:4 MMI coupler 2C has a tapered shapewherein the width increases in a square function from an input end 2CXtoward an output end 2CY thereof. It is to be noted that, in FIG. 36,like elements to those of the second embodiment described above [referto FIG. 22A] are denoted by like reference characters.

In this manner, in the present embodiment, the 2:4 MMI coupler 2C has awidth tapered structure with which the phase difference Δθ of a pair ofsecond optical signals outputted from the 2:4 MMI coupler 2C becomesπ/2+p*π (p is an integer) so that the phase difference of lights to beinputted to the two channels on the input side of the 2:2 MMI coupler 3may be 90 degrees.

In this instance, the 2:4 MMI coupler 2C (inclined 2:4 MMI coupler) hasan input end 2CX of a first width W_(S) and an output end 2CY of asecond width W_(M) different from the first width W_(S) and isconfigured such that the phase difference Δθ between a pair of secondoptical signals becomes π/2+p*π (p is an integer). It is to be notedthat the values of the width W_(S) and the width W_(M) are differentfrom those in the second embodiment described hereinabove.

Where the width taper varies in a square function as illustrated in FIG.36, the propagation constant difference between the fundamental mode andan arbitrary higher-order mode varies locally.

In this instance, the net phase shift (Δρ) in the MMI region is similarto that given by the expression (9).

However, since, in the present embodiment, a square function taperedshape is used and the variation of the width taper is different, thewidth taper function W_(M)(z) is given by the following expression (14):

$\begin{matrix}{{W_{M}(z)} = {W_{S} + {\left( {W_{M} - W_{S}} \right) \cdot \left( \frac{z}{L_{M\; 24}} \right)^{2}}}} & (14)\end{matrix}$

From the expressions (9) and (14) above, the propagation constantdifference <β₀−β_(v)> between the fundamental mode and an arbitraryhigher-order mode is represented by the following expression:

$\begin{matrix}{{\langle{\beta_{0} - \beta_{v}}\rangle} = {\frac{{v\left( {v + 2} \right)}\pi \; \lambda}{4\; n_{r}W_{M}^{2}}\chi^{SQ}}} & (15) \\{\chi^{SQ} = {\frac{W_{M}}{2}\left( {\frac{1}{W_{S}} + \frac{W_{M}{\tanh^{- 1}\left\lbrack \sqrt{\frac{{- W_{M}} + W_{S}}{W_{S}}} \right\rbrack}}{W_{S}\sqrt{W_{S}}\sqrt{{- W_{M}} + W_{S}}}} \right)}} & (16)\end{matrix}$

where χ^(ST) is a proportionality constant which relies upon the taperedshape.

From the expressions (15) and (16), the beat length L_(π) ^(SQ) of the2:4 MMI coupler which has such a square function tapered shape can berepresented by the following expression (17):

$\begin{matrix}{L_{\pi}^{SQ} = \frac{L_{\pi}}{\chi^{SQ}}} & (17)\end{matrix}$

Accordingly, the beat length L_(π) ^(SQ) of the 2:4 MMI coupler 2C whichhas such a square function tapered shape as illustrated in FIG. 36decreases as the proportionally constant χ^(SQ) increases with respectto the beat length L_(π) of the 2:4 MMI coupler (refer to FIG. 6) whichdoes not have a tapered shape. It is to be noted that the MMI width ofthe 2:4 MMI coupler (refer to FIG. 6) which does not have a taperedshape is set equal to the width W_(M) of the output end 2CY of the 2:4MMI coupler 2C which has a square function tapered shape of the presentembodiment. Here, 1/χ^(SQ) is considered a parameter which representsthe shortening ratio (the reduction ratio) of the MMI length L_(M24) ofthe 2:4 MMI coupler 2C having a square function tapered shape of thepresent embodiment with respect to the MMI length of the 2:4 MMI coupler(refer to FIG. 6) which does not have a tapered shape. It is to be notedthat the value of the MMI length L_(M24) is different from that in thesecond embodiment described hereinabove.

Meanwhile, the phase variation of the 2:4 MMI coupler 2C relies upon thetapered shape which varies in a square function.

Therefore, where Δφ is −π/2 and +π/2, 90-degree hybrid operation isobtained with certainty by setting χ^(SQ) such that the phase differenceΔθ between a pair of second optical signals to be outputted from the 2:4MMI coupler 2C becomes equal to π/2+p*π (p is an integer). In otherwords, by setting χ^(SQ) to a proper value, such an optical hybridcircuit as shown in FIG. 36 functions as a 90-degree hybrid. Alsoshortening of the 2:4 MMI length L_(M24) can be achieved.

FIG. 37 illustrates a relationship between the rate of change of the MMIwidth of the 2:4 MMI coupler 2C, that is, a relationship between theratio W_(M)/W_(S) between the width W_(S) of the input end 2CX and thewidth W_(M) of the output end 2CY of the 2:4 MMI coupler 2C, and theabsolute value |Δθ| of the inter-channel phase difference of the outputsignals. It is to be noted that, in FIG. 37, the absolute value |Δθ| ofthe inter-channel phase difference of the output signals is an absolutevalue of the inter-channel phase difference between the output signals(a pair of second optical signals) outputted from the third and fourthoutput channels of the 2:4 MMI coupler 2C.

Meanwhile, FIG. 37 illustrates a relationship between the rateW_(M)/W_(S) of change of the MMI width of the 2:4 MMI coupler 2C and thereduction ratio of the 2:4 MMI length L_(M24), that is, 1/χ^(SQ) (thatis, W_(S)/W_(M)).

It is to be noted that, where the value of W_(M)/W_(S) is 1, the 2:4 MMIcoupler does not have a tapered shape (refer to FIG. 6). Further, thewidth W_(M) of the output end 2CY of the 2:4 MMI coupler 2C is set equal(fixed) while the width W_(S) of the input end 2CX is varied withrespect to the 2:4 MMI coupler (FIG. 6) which does not have a taperedshape. It is to be noted that alternatively the width W_(S) of the inputend 2CX of the 2:4 MMI coupler 2C may be fixed while the width W_(M) ofthe output end 2CY is varied.

As shown in FIGS. 37 and 38, it can be recognized that, as the value ofW_(M)/W_(S) increases, the value of the absolute value |Δθ| of theinter-channel phase difference of the output signals increases linearlyand the value of 1/χ^(SQ) decreases (in other words, the MMI lengthL_(M24) is shortened).

Further, since a square function tapered shape is used, the rate ofchange of the value of |Δθ| with respect to the value of W_(M)/W_(S) isgreater than that where a linear function tapered shape is used (referto FIGS. 24 and 25) as illustrated in FIGS. 37 and 38.

Therefore, the absolute value |Δθ| of the inter-channel phase differenceof the output signals can be set to π/2 which is the desired value bysetting of the value of the width W_(S) of the input end 2CX of the 2:4MMI coupler 2C (that is, of the value of W_(M)/W_(S)).

Here, where the absolute value |Δθ| of the inter-channel phasedifference of the output signals, the value of W_(M)/W_(S) isapproximately 1.5 as illustrated in FIG. 37, and where the value ofW_(M)/W_(S) is approximately 1.5, the value of 1/χ^(SQ) is approximately0.58 as illustrated in FIG. 38. In this instance, the value of χ^(ST) isapproximately 1.74.

Accordingly, the MMI length L_(m24) of the 2:4 MMI coupler 2C having asquare function tapered shape is equal to 1/χ^(SQ) time, that is, toapproximately 0.58 times, the MMI length of the 2:4 MMI coupler (referto FIG. 6) which does not have a tapered shape. In other words, the MMIlength L_(M24) of the 2:4 MMI coupler 2C having a square functiontapered shape is shortened by approximately 42% in comparison with theMMI length of the 2:4 MMI coupler (refer to FIG. 6) which does not havea tapered shape.

Further, the relationships illustrated in FIGS. 37 and 38 is satisfiedwith regard to an arbitrary width W_(S) of the input end 2CX and anarbitrary width W_(M) of the output end 2CY. In particular, if thecondition of χ^(SQ)=approximately 1.74 is satisfied with regard to anarbitrary width W_(S) of the input end 2CX and an arbitrary width W_(M)of the output end 2CY, then |Δθ|=π/2 is satisfied, and 90-degree hybridoperation is obtained with certainty using the 2:4 MMI coupler 2C havingsuch a square function tapered shape as illustrated in FIG. 36.

In particular, where the width W_(M) of the input end 2CX of the 2:4 MMIcoupler 2C having a square function tapered shape is approximately 33μm, the width W_(S) of the input end 2CX is decided to approximately 21μm from the condition of the absolute value |Δθ| of the inter-channelphase difference of the output signals=π/2 (refer to FIG. 37). After thewidth W_(M) of the output end 2CY and the width W_(S) of the input end2CX of the 2:4 MMI coupler 2C are decided in this manner, the length L₂₄of the 2:4 MMI coupler 2C is decided to approximately 440 μm from thecondition of 1/χ^(SQ)=approximately 0.58, that is, χ^(SQ)=approximately1.74. The square function tapered shape of the 2:4 MMI coupler 2C isdefined by an expression [width taper function W_(M) (z)] obtained bysubstituting the decided values into the expression (14) givenhereinabove.

If it is assumed that the width W_(M) of the output end 2CY of the 2:4MMI coupler 2C having a square function tapered shape is set toapproximately 26.4 μm (which corresponds to approximately 80% ofapproximately 33 μm), then the desired width W_(S) of the input end 2CXis approximately 16.8 μm (which corresponds to approximately 80% ofapproximately 21 μm).

Since the square function tapered shape is defined in this manner andthe MMI width is modulated with a predetermined taper function, thephase difference Δθ between a pair of second optical signals outputtedfrom the 2:4 MMI coupler 2C becomes π/2+p*π (p is an integer).Therefore, the present optical hybrid circuit 1X outputs a pair of firstoptical signals (S−L and S+L) having an in-phase relationship with eachother and a pair of third optical signals (S−jL and S+jL) having aquadrature phase relationship with the pair of first optical signals,and 90-degree hybrid operation is obtained with certainty. In otherwords, QPSK signal light is converted into a pair of first opticalsignals which include only an in-phase component (I-component) and apair of third optical signals which include only a quadrature phasecomponent (Q-component) by the present optical hybrid circuit 1X, and90-degree hybrid operation is obtained with certainty.

Here, the pair of first optical signals having an in-phase relationshipwith each other, that is, the pair of first optical signals whichinclude only an in-phase component, are a pair of optical signals whosephases are displaced by 180 degrees from each other. Meanwhile, the pairof third optical signals having a quadrature phase relationship with thepair of first optical signals, that is, the pair of third opticalsignals which include only a quadrature phase component, are a pair ofoptical signals whose phases are displaced by 90 degrees from those ofthe pair of first optical signals. It is to be noted that the pair ofthird optical signals are a pair of optical signals whose phases aredisplaced by 180 degrees from each other.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 36 indicate what relative relationship the phase of the LO light(L) has with reference to the phase of the signal light (S). Here, S−Land S+L indicate that they have a phase relationship displaced by 180degrees from each other, and S+jL and S−jL have a phase relationshipdisplaced by 90 degrees with respect to S+L and S−L, respectively.

It is to be noted that, since details of the other part are similar tothose of the second embodiment and the modifications [refer to, forexample, FIGS. 31 to 35] described hereinabove, overlapping descriptionof them is omitted herein.

Accordingly, the optical hybrid circuit according to the presentembodiment is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and a 90-degree hybrid suitable for compactness and monolithicintegration can be implemented similarly to the second embodimentdescribed hereinabove.

Further, since an intersecting portion of optical waveguides whichcannot be avoided with the conventional 90-degree hybrid [refer to FIG.50A] which uses a 4:4 MMI coupler is not required, there is an advantagealso in that excessive loss can be suppressed to the minimum. Further,since the phase relationship of the four output signals can be madesimilar to that of the conventional 90-degree hybrid [refer to FIGS. 48Aand 49A], the optical hybrid circuit according to the present embodimentis superior also in compatibility with 90-degree hybrids which are usedin coherent optical receivers, coherent detection systems and so forthat present.

Fourth Embodiment

Now, an optical hybrid circuit according to a fourth embodiment isdescribed with reference to FIGS. 39 to 41.

The optical hybrid circuit according to the present embodiment isdifferent from that of the second embodiment described hereinabove inthe tapered shape of the 2:4 MMI coupler.

In particular, in the present optical hybrid circuit, the 2:4 MMIcoupler 2D has a tapered shape (exponential function tapered shape;width tapered structure) wherein the width (MMI width; waveguide width)thereof varies in an exponential function toward the propagationdirection as illustrated in FIG. 39. Here, the 2:4 MMI coupler 2D has atapered shape wherein the width increases in an exponential functionfrom an input end 2DX toward an output end 2DY thereof. It is to benoted that, in FIG. 39, like elements to those of the second embodimentdescribed above [refer to FIG. 22A] are denoted by like referencecharacters.

In this manner, in the present embodiment, the 2:4 MMI coupler 2D has awidth tapered structure with which the phase difference Δθ between apair of second optical signals outputted from the 2:4 MMI coupler 2Dbecomes π/2+p*π (p is an integer) so that the phase difference betweenlights to be inputted to the two channels on the input side of the 2:2MMI coupler 3 may be 90 degrees.

In this instance, the 2:4 MMI coupler 2D (inclined 2:4 MMI coupler) hasan input end 2DX of a first width W_(S) and an output end 2DY of asecond width W_(M) different from the first width W_(S) and isconfigured such that the phase difference Δθ between a pair of secondoptical signals becomes π/2+p*π (p is an integer). It is to be notedthat the values of the width W_(S) and the width W_(M) are differentfrom those in the second embodiment described hereinabove.

Where the width taper varies in an exponential function as illustratedin FIG. 39, the propagation constant difference between the fundamentalmode and an arbitrary higher-order mode varies locally.

In this instance, the net phase shift (Δρ) in the MMI region is similarto that given by the expression (9).

However, since, in the present embodiment, an exponential functiontapered shape is used and the variation of the width taper is different,the width taper function W_(M)(z) is given by the following expression(18):

$\begin{matrix}{{W_{M}(z)} = {W_{S} + {\left( {W_{M} - W_{S}} \right) \cdot \left( \frac{{{Exp}\left( {z/L_{M\; 24}} \right)} - 1}{e - 1} \right)}}} & (18)\end{matrix}$

From the expressions (9) and (18) above, the propagation constantdifference <β₀−β_(v)> between the fundamental mode and an arbitraryhigher-order mode is represented by the following expression:

$\begin{matrix}{{\langle{\beta_{0} - \beta_{v}}\rangle} = {\frac{{v\left( {v + 2} \right)}\pi \; \lambda}{4\; n_{r}W_{M}^{2}}\chi^{EXP}}} & (19) \\{\chi^{EXP} = \frac{\left( {e - 1} \right){W_{M}\begin{pmatrix}{W_{M}^{2} - {2\; W_{S}W_{M}} + {e \cdot W_{S}^{2}} -} \\{\left( {e - 1} \right)W_{S}W_{M}{\log \left( {W_{M}/W_{S}} \right)}}\end{pmatrix}}}{W_{S}\left( {W_{M} - {e \cdot W_{S}}} \right)}} & (20)\end{matrix}$

Where χ^(EXP) is a proportionality constant which relies upon thetapered shape.

From the expressions (19) and (20), the beat length L_(π) ^(EXP) of the2:4 MMI coupler which has an exponential function tapered shape can berepresented by the following expression (21):

$\begin{matrix}{L_{\pi}^{EXP} = \frac{L_{\pi}}{\chi^{EXP}}} & (21)\end{matrix}$

Accordingly, the beat length L_(π) ^(EXP) of the 2:4 MMI coupler 2Dwhich has such an exponential function tapered shape as illustrated inFIG. 39 decreases as the proportionality constant χ^(EXP) increases withrespect to the beat length L_(π) of the 2:4 MMI coupler (refer to FIG.6) which does not have a tapered shape. It is to be noted that the MMIwidth of the 2:4 MMI coupler (refer to FIG. 6) which does not have atapered shape is set equal to the width W_(M) of the output end 2DY ofthe 2:4 MMI coupler 2D which has an exponential function tapered shapeof the present embodiment. Here, 1/χ^(EXP) is considered a parameterwhich represents the shortening ratio (the reduction ratio) of the MMIlength L_(M24) of the 2:4 MMI coupler 2D having an exponential functiontapered shape of the present embodiment with respect to the MMI lengthof the 2:4 MMI coupler (refer to FIG. 6) which does not have a taperedshape. It is to be noted that the value of the MMI length L_(M24) isdifferent from that in the second embodiment described hereinabove.

Meanwhile, the phase shift in the 2:4 MMI coupler 2D relies upon thetapered shape which varies in an exponential function.

Therefore, where Δφ is −π/2 and +π/2, 90-degree hybrid operation isobtained with certainty by setting χ^(EXP) such that the phasedifference Δθ between a pair of second optical signals to be outputtedfrom the 2:4 MMI coupler 2D becomes equal to π/2+p*π (p is an integer).In other words, by setting χ^(EXP) to a proper value, such an opticalhybrid circuit as shown in FIG. 39 functions as a 90-degree hybrid. Alsoshortening of the 2:4 MMI length L_(M24) can be achieved.

FIG. 40 illustrates a relationship between the rate of change of the MMIwidth of the 2:4 MMI coupler 2D, that is, the ratio W_(M)/W_(S) betweenthe width W_(S) of the input end 2DX and the width W_(M) of the outputend 2DY of the 2:4 MMI coupler 2D, and the absolute value |Δθ| of theinter-channel phase difference of the output signals. It is to be notedthat, in FIG. 40, the absolute value |Δθ| of the inter-channel phasedifference of the output signals is an absolute value of theinter-channel phase difference of the output signals (a pair of secondoptical signals) outputted from the third and fourth output channels ofthe 2:4 MMI coupler 2D.

Meanwhile, FIG. 41 illustrates a relationship between the rateW_(M)/W_(S) of change of the MMI width of the 2:4 MMI coupler 2D and thereduction ratio of the 2:4 MMI length L_(M24), that is, 1/χ^(EXP) (thatis, W_(S)/W_(M)).

It is to be noted that, where the value of W_(M)/W_(S) is 1, the 2:4 MMIcoupler does not have a tapered shape (refer to FIG. 6). Further, thewidth W_(M) of the output end 2DY of the 2:4 MMI coupler 2D is set equal(fixed) while the width W_(S) of the input end 2DX is varied withrespect to the 2:4 MMI coupler (refer to FIG. 6) which does not have atapered shape. It is to be noted that alternatively the width W_(S) ofthe input end 2DX of the 2:4 MMI coupler 2D may be fixed while the widthW_(M) of the output end 2DY is varied.

As shown in FIGS. 40 and 41, it can be recognized that, as the value ofW_(M)/W_(S) increases, the value of the absolute value |Δθ| of theinter-channel phase difference of the output signals increases linearlyand the value of 1/χ^(EXP) decreases (in other words, the MMI lengthL_(M24) is shortened).

Further, since an exponential function tapered shape is used, the rateof change of the value of |Δθ| with respect to the value of W_(M)/W_(S)is greater than that where a linear function tapered shape is used(refer to FIGS. 24 and 25) as illustrated in FIGS. 40 and 41.

Therefore, the absolute value |Δθ| of the inter-channel phase differenceof the output signals can be set to π/2 which is the desired value bysetting of the value of the width W_(S) of the input end 2DX of the 2:4MMI coupler 2D (that is, of the value of W_(M)/W_(S)).

Here, where the absolute value |Δθ| of the inter-channel phasedifference of the output signal, the value of W_(M)/W_(S) isapproximately 1.68, as illustrated in FIG. 40, and where the value ofW_(M)/W_(S) is 1.68, the value of χ^(EXP) is approximately 0.56 asillustrated in FIG. 41. In this instance, χ^(EXP) is approximately 1.79.

Accordingly, the MMI length L_(M24) of the 2:4 MMI coupler 2D having anexponential function tapered shape is equal to 1/χ^(EXP) time, that is,to approximately 0.56 times, the MMI length of the 2:4 MMI coupler(refer to FIG. 6) which does not have a tapered shape. In other words,the MMI length L_(M24) of the 2:4 MMI coupler 2D having an exponentialfunction tapered shape is shortened by approximately 44% in comparisonwith the MMI length of the 2:4 MMI coupler (refer to FIG. 6) which doesnot have a tapered shape.

Further, the relationships illustrated in FIGS. 40 and 41 are satisfiedwith regard to an arbitrary width W_(S) of the input end 2DX and anarbitrary width W_(M) of the output end 2DY. In particular, if thecondition of χ^(EXP)=approximately 1.79 is satisfied with regard to anarbitrary width W_(S) of the input end 2DX and an arbitrary width W_(M)of the output end 2DY, then |Δθ|=π/2 is satisfied, and 90-degree hybridoperation is obtained with certainty using the 2:4 MMI coupler 2D havingsuch an exponential function tapered shape as illustrated in FIG. 39.

In particular, where the width W_(M) of the output end 2DY of the 2:4MMI coupler 2D having an exponential function tapered shape is 33 μm,the width W_(S) of the input end 2DX is decided to 20 μm from thecondition that the absolute value |Δθ| of the inter-channel phasedifference of the output signals is π/2 (|Δθ|=π/2) (refer to FIG. 40).After the width W_(M) of the output end 2DY and the width W_(S) of theinput end 2DX of the 2:4 MMI coupler 2D are decided in this manner, thelength L_(M24) of the 2:4 MMI coupler 2D is decided to approximately 423μm from the condition of 1/χ^(EXP)=approximately 0.56, that is,χ^(EXP)=approximately 1.79. The exponential function tapered shape ofthe 2:4 MMI coupler 2D is defined by an expression [width taper functionW_(M)(z)] obtained by substituting the decided values into theexpression (18) given hereinabove.

Since the exponential function tapered shape is defined in this mannerand the MMI width is modulated with a predetermined taper function, thephase difference Δθ between a pair of second optical signals outputtedfrom the 2:4 MMI coupler 2D becomes π/2+p*π (p is an integer).Therefore, the present optical hybrid circuit 1X outputs a pair of firstoptical signals (S−L and S+L) having an in-phase relationship with eachother and a pair of third optical signals (S−jL and S+jL) having aquadrature phase relationship with the pair of first optical signals,and 90-degree hybrid operation is obtained with certainty. In otherwords, QPSK signal light is converted into a pair of first opticalsignals which include only an in-phase component (I-component) and apair of third optical signals which include only a quadrature phasecomponent (Q-component) by the present optical hybrid circuit 1X, and90-degree hybrid operation is obtained with certainty.

Here, the pair of first optical signals having an in-phase relationshipwith each other, that is, the pair of first optical signals whichinclude only an in-phase component, are a pair of optical signals whosephases are displaced by 180 degrees from each other. Meanwhile, the pairof third optical signals having a quadrature phase relationship with thepair of first optical signals, that is, the pair of third opticalsignals which include only a quadrature phase component, are a pair ofoptical signals whose phases are displaced by 90 degrees from those ofthe pair of first optical signals. It is to be noted that the pair ofthird optical signals are a pair of optical signals whose phases aredisplaced by 180 degrees from each other.

It is to be noted that reference characters S−L, S+L, S+jL and S−jL inFIG. 39 indicate what relative relationship the phase of the LO light(L) has with reference to the phase of the signal light (S). Here, S−Land S+L indicate that they have a phase relationship displaced by 180degrees from each other, and S+jL and S−jL have a phase relationshipdisplaced by 90 degrees with respect to S+L and S−L, respectively.

It is to be noted that, since details of the other part are similar tothose of the second embodiment and the modifications [refer to, forexample, FIGS. 31 to 35] described hereinabove, overlapping descriptionof them is omitted herein.

Accordingly, the optical hybrid circuit according to the presentembodiment is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and a 90-degree hybrid suitable for compactness and monolithicintegration can be implemented similarly to the second embodimentdescribed hereinabove.

Further, since an intersecting portion of optical waveguides whichcannot be avoided with the conventional 90-degree hybrid [refer to FIG.50A] which uses a 4:4 MMI coupler is not required, there is an advantagealso in that excessive loss can be suppressed to the minimum. Further,since the phase relationship of the four output signals can be madesimilar to that of the conventional 90-degree hybrid [refer to FIGS. 48Aand 49A], the optical hybrid circuit according to the present embodimentis superior also in compatibility with 90-degree hybrids which are usedin coherent optical receivers, coherent detection systems and so forthat present.

It is to be noted that the taper function which defines the taper faceof 2:4 MMI coupler which composes the optical hybrid circuit 1Xaccording to the second to fourth embodiments described above must bedefined only such that the phase difference between a pair of firstoptical signals or between a pair of second optical signals becomesπ/2+p*π (p is an integer). Therefore, the tapered shape of the 2:4 MMIcoupler is not limited to any of the liner function tapered shape in thesecond embodiment, the square function tapered shape in the thirdembodiment and the exponential function tapered shape in the fourthembodiment, but, for example, a square root function tapered shape, asine wave function tapered shape or a tapered shape of a combination ofany ones of the functions may be used.

Fifth Embodiment

Now, an optical receiver, an optical transceiver and an opticalreception method according to a fifth embodiment are described withreference to FIG. 42.

The optical receiver according to the present embodiment is a coherentoptical receiver 20 which includes the optical hybrid circuit 1 of anyof the first embodiment and the modifications to the first embodiment[90-degree hybrid for a QPSK signal: refer to FIGS. 1A, 3A and 16 to21B]. The coherent optical receiver 20 converts an optical signalidentified by the 90-degree hybrid 1 and carries out a digital signalprocessing.

To this end, the present coherent optical receiver 20 includes, asillustrated in FIG. 42, the optical hybrid circuit 1 of the firstembodiment described hereinabove, a pair of photodiodes (photoelectricconversion sections) 21A and 21B, a pair of trans-impedence amplifier(TIA) 27A, 27B, a pair of AD conversion circuits (AD conversionsections) 22A and 22B, and a digital arithmetic circuit (digitalarithmetic section) 23.

Here, the present optical hybrid circuit 1 includes an MMI coupler 2 forconverting QPSK signal light into a pair of first optical signals havingan in-phase relationship with each other and a pair of second opticalsignals having an in-phase relationship with each other, and an opticalcoupler 3 for converting the first optical signals or the second opticalsignals into a pair of third optical signals having a quadrature phaserelationship with the first or second optical signals [refer to FIGS.1A, 3A and 16 to 21(B)].

Here, the MMI coupler 2 is a 2:4 MMI coupler. Meanwhile, the opticalcoupler 3 is a 2:2 MMI coupler. The optical hybrid circuit 1 is formedfrom an optical semiconductor device.

In the present embodiment, QPSK signal light is inputted to one ofchannels on the input side of the 2:4 MMI coupler 2 of the opticalhybrid circuit 1 and LO light is inputted to the other channel on theinput side of the 2:4 MMI coupler 2 as illustrated in FIG. 42. In otherwords, the one channel on the input side of the 2:4 MMI coupler 2 of theoptical hybrid circuit 1 is a channel for inputting QPSK signal light.Meanwhile, the other channel on the input side of the 2:4 MMI coupler 2of the optical hybrid circuit 1 is a channel for inputting LO light.

Therefore, the coherent optical receiver 20 further includes a localoscillation light generation section (LO light source) 24 for inputtingLO light to the other channel on the input side of the 2:4 MMI coupler 2of the optical hybrid circuit 1.

Thus, if QPSK signal light (QPSK signal pulse) and LO light synchronizedin time with the QPSK signal light are inputted to the optical hybridcircuit 1, then one of output forms having different branching rationsfrom each other is obtained in response to the relative phase differenceΔφ between the QPSK signal light and the LO light. Here, where therelative phase difference Δφ is 0, n, −π/2 and +π/2, the outputintensity ratio of the optical hybrid circuit 1 is 0:2:1:1, 2:0:1:1,1:1:2:0 and 1:1:0:2 [refer to FIGS. 9A to 9D], respectively.

The photodiodes 21A and 21B are photodiodes for photoelectricallyconverting pairs of optical signals outputted from the multimodeinterference coupler 2 and the optical coupler 3 of the optical hybridcircuit 1 into analog electric signals (analog current signals).

Here, for photoelectric conversion and signal demodulation, thedifferential photodiodes (BPDs) 21A and 21B are provided at thesucceeding stage of the optical hybrid circuit 1. Here, each of the BPDs21A and 21B includes two photodiodes (PDs). If an optical signal isinputted only to one of the PDs in each of the BPDs 21A and 21B, thencurrent corresponding to “1” flows, but if an optical signal is inputtedonly to the other PD, current corresponding to “−1” flows. However, ifan optical signal is inputted to both of the PDs simultaneously, then nocurrent flows.

Therefore, if optical signals having different output intensity ratiosare inputted from the optical hybrid circuit 1 to the two BPDs 21A and21B in response to the relative phase difference Δφ, then electricsignals of different patterns are outputted from the two BPDs 21A and21B. In particular, phase information of the QPSK signal light isidentified and converted into electric signals of different patterns bythe two BPDs 21A and 21B.

In particular, the first BPD 21A is connected to the first and secondchannels on the output side of the optical hybrid circuit 1, and thesecond BPD 21B is connected to the third and fourth channels on theoutput side of the optical hybrid circuit 1. In other words, the firstBPD 21A is connected to the first and second channels (a pair of firstoutput channels neighboring with each other) from which a pair of firstoptical signals having an in-phase relationship with each other areoutputted, and the second BPD 21B is connected to the third and fourthchannels (a pair of second output channels neighboring with each other)to which a pair of second optical signals having an in-phaserelationship with each other (but having a quadrature phase relationshipwith the first optical signals) are outputted.

Trans-impedance amplifiers 27A and 27B are provided between thephotodiodes 21A and 21B and the AD conversion circuits 22A and 22B,respectively. More specifically, the trans-impedance amplifiers 27A and27B are connected to the photodiodes 21A and 21B, and to the ADconversion circuits 22A and 22B. The trans-impedance amplifiers 27A and27B are adapted to convert the analog current signals output from thephotodiodes 21A and 21B into analog voltage signals (analog electricsignals).

The AD conversion circuits 22A and 22B are AD conversion circuits thatconvert the analog electric signals that are output from the photodiodes21A and 21B and then undergo the conversion at the trans-impedanceamplifiers 27A and 27B, into digital electric signals. Morespecifically, the AD conversion circuits 22A and 22B are adapted toconvert the analog electric signals that are output from thetrans-impedance amplifiers 27A and 27B, into digital electric signals.

The digital arithmetic circuit 23 is a digital arithmetic circuit(digital signal processing circuit) which uses the digital electricsignals outputted from the AD conversion circuits 22A and 22B to executean arithmetic operation for estimating information of reception signallight.

Since the present optical receiver 20 is configured in such a manner asdescribed above, it receives an optical signal in the following manner(light receiving method).

In particular, the multimode interference coupler (here the 2:4 MMIcoupler 2) of the optical hybrid circuit 1 is used to convert QPSKsignal light into a pair of first optical signals having an in-phaserelationship with each other and a pair of second optical signals havingan in-phase relationship with each other. Then, the optical coupler(here the 2:2 MMI coupler 3) is used to convert the first opticalsignals or the second optical signals into a pair of third opticalsignals having a quadrature phase relationship with the first or secondoptical signals. Then, the first optical signals or second opticalsignals and the third optical signals are received.

It is to be noted that, since details of the optical hybrid circuit 1are similar to those of the first embodiment and the modifications tothe first embodiment described hereinabove, description of the same isomitted here.

Accordingly, the optical receiver according to the present embodiment isadvantageous in that it exhibits a low wavelength dependency, a lowphase displacement characteristic and low insertion loss and a 90-degreehybrid suitable for compactioness and monolithic integration can beimplemented.

It is to be noted that, while, in the description of the embodiment andthe modifications described hereinabove, the optical receiver isdescribed taking an optical receiver including any of the optical hybridcircuits of the first embodiment and the modifications to the firstembodiment described above as an example, the optical receiver is notlimited to this. For example, the optical receiver may be configured asan optical receiver 20X which includes one of the optical hybridcircuits 1X of the second to fourth embodiments and the modifications tothem described hereinabove as illustrated in FIG. 43. It is to be notedthat, in FIG. 43, like elements to those of the embodiment describedabove [refer to FIG. 42] are denoted by like reference characters.

Further, while the description of the embodiment and the modificationsdescribed above is given taking an optical receiver as an example, theoptical receiver is not limited to this, but also it is possible toconfigure an optical transceiver which includes the configuration of theoptical receiver of the embodiment described hereinabove.

Further, while, in the embodiments and the modifications describedabove, the optical hybrid circuit 1 or 1X is formed from an opticalsemiconductor device which includes the MMI coupler 2, 2A, 2B, 2C or 2Dand the optical coupler 3, it is not limited to this. For example, theoptical semiconductor device which includes the MMI coupler 2, 2A, 2B,2C or 2D and the optical coupler 3 may additionally include photodiodes(here BPDs) 21A and 21B integrated therein. In short, the MMI coupler 2,2A, 2B, 2C or 2D, the optical coupler 3 and the photodiodes 21A and 21B(here the BPDs) may be monolithically integrated.

Sixth Embodiment

Now, an optical hybrid circuit according to a sixth embodiment isdescribed with reference to FIG. 44.

The optical hybrid circuit according to the present embodiment isdifferent from that of the first embodiment described hereinabove inthat, while, in the optical hybrid circuit of the first embodiment, QPSKsignal light and LO light are inputted in a synchronized relationshipwith each other in time, in the optical hybrid circuit according to thepresent embodiment, a differential quadrature phase shift keying (DQPSK)signal is inputted.

In particular, the present optical hybrid circuit is a 90-degree hybridcircuit (hereinafter referred to also as 90-degree hybrid) used foridentification of phase modulation information of a DQPSK signal in anoptical transmission system.

To this end, as illustrated in FIG. 44, the present optical hybridcircuit 1A includes, in addition to the configuration of the opticalhybrid circuit 1 of the first embodiment described hereinabove, a lightdelay circuit 5, and a 1:2 optical coupler 6 having one channel on theinput side thereof and having two channels on the output side thereof.In other words, the present optical hybrid circuit 1A is configured suchthat the 1:2 optical coupler 6 is connected in a cascade connection atthe preceding stage (front end portion) of the 2:4 MMI coupler 2included in the optical hybrid circuit 1 of the first embodimentdescribed hereinabove through the light delay circuit 5. It is to benoted that, similarly to the first embodiment described hereinabove, theoptical hybrid circuit 1 is formed from an optical semiconductor devicewhich includes the MMI coupler 2 and the optical coupler 3. It is to benoted that, in FIG. 44, like elements to those of the embodimentdescribed above [refer to FIG. 1A] are denoted by like referencecharacters.

Here, the light delay circuit 5 is connected to one of the channels onthe input side of the 2:4 MMI coupler 2 included in the optical hybridcircuit 1 of the first embodiment described hereinabove.

The 1:2 optical coupler 6 is connected to the light delay circuit 5 andthe other channel on the input side of the 2:4 MMI coupler 2. Here, the1:2 optical coupler 6 is a 1:2 MMI coupler.

In particular, the length of one waveguide which connects one of theinput channels of the 2:4 MMI coupler 2 and one of the output channelsof the 1:2 optical coupler 6 to each other is set greater than thelength of the other optical waveguide which connects the other inputchannel of the 2:4 MMI coupler 2 and the other output channels of the1:2 optical coupler 6 to each other.

In particular, the two optical waveguides (arms) which connect the twoinput ports of the 2:4 MMI coupler 2 and the two output ports of the 1:2MMI coupler 6 to each other are different in length (optical pathlength) from each other.

Here, the length of one of the optical waveguides is made longer toprovide an optical path length difference which corresponds to delay ofone bit of a DQPSK signal pulse. To this end, the light delay circuit 5is one of the optical waveguides which is connected to one of the inputchannels of the 2:4 MMI coupler 2 includes in the optical hybrid circuit1 of the first embodiment described hereinabove.

Thus, DQPSK signal light is inputted to the channel on the input side ofthe 1:2 MMI coupler 6. Therefore, the channel on the input side of the1:2 MMI coupler 6 is an input channel for inputting DQPSK signal light.The DQPSK signal is branched into two paths through the 1:2 MMI coupler6, and one of the branched DQPSK signal lights is delayed by one bit bythe light delay circuit 5. Then, the two DQPSK signal lights areinputted to the 2:4 MMI coupler 2 in synchronism with each other intime. In this instance, the relative phase difference between the DQPSKsignal lights individually inputted to the two input channels of the 2:4MMI coupler 2 is any one of the four kinds of the relative phasedifference Δφ described hereinabove in connection with the firstembodiment (refer to FIGS. 9A to 9D). Therefore, output forms havingdifferent branching ratios from each other are obtained by the circuitconfiguration including the 2:4 MMI coupler and the succeeding circuitelements similar to those of the first embodiment described hereinabove.Accordingly, also the optical hybrid circuit 1A functions as a 90-degreehybrid similarly as in the case of the first embodiment describedhereinabove.

It is to be noted that, since details of the other part are similar tothose of the first embodiment described hereinabove, overlappingdescription of them is omitted herein. Here, when the first embodimentdescribed hereinabove is applied to the present embodiment, two DQPSKsignal lights having a relative phase difference Δφ maybe applied inplace of the QPSK signal light and the LO light.

Accordingly, the optical hybrid circuit according to the presentembodiment is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and a 90-degree hybrid suitable for compactness and monolithicintegration can be implemented similarly as in the first embodimentdescribed hereinabove.

Further, since an intersecting portion of optical waveguides whichcannot be avoided with the conventional 90-degree hybrid [refer to FIG.50A] which uses a 4:4 MMI coupler is not required, there is an advantagealso in that excessive loss can be suppressed to the minimum. Further,since the phase relationship of the four output signals can be madesimilar to that of the conventional 90-degree hybrid [refer to FIGS. 48Aand 49A], the optical hybrid circuit according to the present embodimentis superior also in compatibility with 90-degree hybrids which are usedin coherent optical receivers, coherent detection systems and so forthat present.

It is to be noted that, while, in the embodiment described above, a 1:2MMI coupler is used as the 1:2 optical coupler 6 provided at thepreceding stage to the 2:4 MMI coupler 2, the 1:2 optical coupler 6 isnot limited to this. For example, it is possible to use a Y branchingcoupler, a 2:2 directional coupler or the like in place of the 1:2 MMIcoupler. Also in this instance, 90-degree hybrid operation can beobtained similarly as in the case of the embodiment described above.

Further, while, in the embodiment described above, the optical hybridcircuit 1A is configured such that it includes an optical semiconductordevice which includes the MMI coupler 2 and the optical coupler 3similarly as in the first embodiment described hereinabove, the opticalhybrid circuit 1A is not limited to this. For example, the opticalhybrid circuit 1A may otherwise be formed from an optical semiconductordevice which includes an MMI coupler 2, an optical coupler 3, a lightdelay circuit 5 and a 1:2 optical coupler 6.

Further, the modifications [refer to FIG. 3A and FIGS. 16 to 21B] to thefirst embodiment described hereinabove can be applied also to theoptical hybrid circuit according to the present embodiment.

Further, while the optical hybrid circuit in the embodiment describedhereinabove includes the optical hybrid circuit of the first embodimentdescribed hereinabove and is a modification to the first embodiment, itmay otherwise include any of the optical hybrid circuits of the secondto fourth embodiments described above such that it is a modification tothe same as illustrated in FIG. 45. In particular, the optical hybridcircuit 1XA includes, in addition to the configuration of the opticalhybrid circuit 1X of any of the second to fourth embodiments describedabove, a light delay circuit 5 and a 1:2 optical coupler 6 which has onechannel on the input side thereof and has two channels on the outputside thereof. Further, the modifications to the second embodimentdescribed hereinabove (refer to, for example, FIGS. 31 to 35) and themodifications to the third and fourth embodiments can be appliedsimilarly also to the present embodiment.

Seventh Embodiment

Now, an optical receiver, an optical transceiver and a light receivingmethod according to a seventh embodiment are described with reference toFIGS. 46 and 47.

The optical receiver according to the present embodiment is a coherentoptical receiver 20A or 20XA including the optical hybrid circuit 1A or1XA (90-degree hybrid for a DQPSK signal; refer to FIGS. 44, 45, 3 and17 to 21B) of any of the sixth embodiment and the modifications to thesixth embodiment described above as illustrated in FIG. 46 or 47. Thecoherent optical receiver 20A or 20XA converts an optical signalidentified by the optical hybrid circuit 1A or 1XA into electric signalsand carries out a digital signal process.

To this end, the optical receiver 20A or 20XA includes the opticalhybrid circuit 1A or 1XA of the sixth embodiment and the modificationsto the sixth embodiment described hereinabove, the photodiodes(photoelectric conversion sections) 21A and 21B, the trans-impedanceamplifier (TIA) 27A, 27B, the AD conversion circuits (AD conversionsections) 22A and 22B, and the digital arithmetic circuit (digitalarithmetic section) 23.

It is to be noted that details of the optical hybrid circuit are similarto those of the sixth embodiment and the modifications to the sixthembodiment described hereinabove [refer to FIGS. 44, 45, 3A and 16 to21B], and therefore, overlapping description of them is omitted herein.Further, since the configuration and the light receiving method of thephotodiodes 21A and 21B, the trans-impedance amplifier (TIA) 27A, 27B,AD conversion circuits 22A and 22B and digital arithmetic circuit 23 aresimilar to that of the fifth embodiment and the modification to thefifth embodiment described above (FIGS. 42 and 43), overlappingdescription of them is omitted herein. However, the optical receiver 20Adoes not include a local oscillation light generation section. Here,when it is tried to apply the fifth embodiment and the modification tothe fifth embodiment described above to the present embodiment, twoDQPSK signal lights having relative phase difference Δφ may be appliedin place of the QPSK signal light and the LO light. It is to be notedthat, in FIGS. 46 and 47, like elements to those of the fifth embodiment[refer to FIG. 42] and the sixth embodiment [refer to FIG. 44] describedhereinabove are denoted by like reference characters.

Accordingly, the optical receiver according to the present embodimentdescribed above is advantageous in that it exhibits a low wavelengthdependency, a low phase displacement characteristic and low insertionloss and an optical receiver including a 90-degree hybrid suitable forcompactness and monolithic integration can be implemented.

It is to be noted that, while the foregoing description of theembodiment described above is given taking an optical receiver as anexample, the application of the invention is not limited to this, andalso it is possible to form an optical transceiver which includes thecomponents of the optical receiver of the embodiment described abovesimilarly to the modification to the fifth embodiment describedhereinabove.

Further, while, in the embodiment described above, the optical hybridcircuit 1A or 1XA is formed from an optical semiconductor device whichincludes an MMI coupler 2, 2A, 2B, 2C or 2D and an optical coupler 3(refer to the sixth embodiment), it is not limited to this. For example,the photodiodes (here, BPDs) 21A and 21B may be integrated in theoptical semiconductor device which includes the MMI coupler 2, 2A, 2B,2C or 2D and the optical coupler 3. In other words, an MMI coupler 2,2A, 2B, 2C or 2D, an optical coupler 3 and photodiodes (here, BPDs) 21Aand 21B may be monolithically integrated.

Further, while the optical hybrid circuit 1A or 1XA in the embodimentdescribed above is formed from an optical semiconductor device whichincludes an MMI coupler 2, 2A, 2B, 2C or 2D, an optical coupler 3, anlight delay circuit 5 and a 1:2 optical coupler 6 (refer to themodification to the sixth embodiment), it is not limited to this. Forexample, the photodiodes (here, BPDs) 21A and 21B may be integrated withthe optical semiconductor device which includes an MMI coupler 2, 2A,2B, 2C or 2D, an optical coupler 3, an light delay circuit 5 and a 1:2optical coupler 6. In other words, an MMI coupler 2, 2A, 2B, 2C or 2D,an optical coupler 3, an light delay circuit 5, a 1:2 optical coupler 6and photodiodes (here, BPDs) 21A and 21B may be monolithicallyintegrated.

[Others]

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment(s) of the presentinvention has(have) been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

1. An optical hybrid circuit comprising: a multimode interferencecoupler including a pair of input channels provided at positionssymmetrical with each other with respect to a center position in awidthwise direction thereof, a pair of first output channels neighboringwith each other for outputting a pair of first optical signals having anin-phase relationship with each other, and a pair of second outputchannels neighboring with each other for outputting a pair of secondoptical signals having an in-phase relationship with each other, themultimode interference coupler being adapted to convert quadrature phaseshift keying signal light or differential quadrature phase shift keyingsignal light into the pair of first optical signals having an in-phaserelationship with each other and the pair of second optical signalshaving an in-phase relationship with each other; and a 2:2 opticalcoupler connected to the first output channels or the second outputchannels and having two channels on the input side and two channels onthe output side, the 2:2 optical coupler being adapted to convert thepair of first optical signals or the pair of second optical signals intoa pair of third optical signals having a quadrature phase relationshipwith the pair of first or second optical signals.
 2. The optical hybridcircuit according to claim 1, wherein one of the pair of first outputchannels or one of the pair of second output channels to which the 2:2optical coupler is connected includes a phase controlling region.
 3. Theoptical hybrid circuit according to claim 2, wherein the phasecontrolling region is a region in which a phase of one of the pair offirst output signals or a phase of one of the pair of second outputsignals is controlled so that the phase difference between the pair offirst optical signals or the phase difference between the pair of secondoptical signals becomes equal to π/2+p*π, p being an integer.
 4. Theoptical hybrid circuit according to claim 1, wherein the multimodeinterference coupler is a 2:4 multimode interference coupler which hastwo channels on the input side thereof and has four channels on theoutput side thereof.
 5. The optical hybrid circuit according to claim 4,wherein the 2:4 multimode interference coupler is a 2:4 multimodeinterference coupler based on paired interference.
 6. The optical hybridcircuit according to claim 1, wherein the multimode interference coupleris a 4:4 multimode interference coupler which has four channels on theinput side thereof and has four channels on the output side thereof, andtwo of the four channels on the input side which are provided atsymmetrical positions with respect to the center position in thewidthwise direction are input channels for inputting light.
 7. Theoptical hybrid circuit according to claim 1, wherein one of the channelson the input side of the multimode interference coupler is an inputchannel for inputting quadrature phase shift keying signal light, andthe other channel on the input side of the multimode interferencecoupler is an input channel for inputting local oscillation light. 8.The optical hybrid circuit according to claim 1, further comprising: alight delay circuit connected one of the channels on the input side ofthe multimode interference coupler; and a 1:2 optical coupler connectedto the light delay circuit and the other channel on the input side ofthe multimode interference coupler and having one channel on the inputside and two channels on the output side; and wherein the channel on theinput side of the 1:2 optical coupler is an input channel for inputtingdifferential quadrature phase shift keying signal light.
 9. An opticalhybrid circuit comprising: a multimode interference coupler including apair of input channels provided at positions symmetrical with each otherwith respect to a center position in a widthwise direction thereof, apair of first output channels neighboring with each other for outputtinga pair of first optical signals having an in-phase relationship witheach other, and a pair of second output channels neighboring with eachother for outputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other; and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals; the multimode interference coupler having an input end of afirst width and an output end of a second width different from the firstwidth such that a phase difference between the pair of first opticalsignals or a phase difference between the pair of second optical signalsbecomes equal to π/2+p*π, p being an integer.
 10. The optical hybridcircuit according to claim 9, wherein the multimode interference couplerhas a shape in which the width thereof varies in a tapered manner in apropagation direction.
 11. The optical hybrid circuit according to claim9, wherein the multimode interference coupler has a tapered shaperepresented by a linear function, a tapered shape represented by asquare function, a tapered shape represented by an exponential function,a tapered shape represented by a square root function, a tampered shaperepresented by a sine wave function or a tapered shape represented by acombination of any ones of the functions.
 12. The optical hybrid circuitaccording to claim 9, wherein the multimode interference coupler and the2:2 optical coupler are connected directly to each other.
 13. Theoptical hybrid circuit according to claim 9, wherein the multimodeinterference coupler is a 2:4 multimode interference coupler which hastwo channels on the input side thereof and has four channels on theoutput side thereof.
 14. The optical hybrid circuit according to claim9, wherein the multimode interference coupler is a 4:4 multimodeinterference coupler which has four channels on the input side thereofand has four channels on the output side thereof, and two of the fourchannels on the input side which are provided at symmetrical positionswith respect to the center position in the widthwise direction are inputchannels for inputting light.
 15. The optical hybrid circuit accordingto claim 9, wherein one of the channels on the input side of themultimode interference coupler is an input channel for inputtingquadrature phase shift keying signal light, and the other channel on theinput side of the multimode interference coupler is an input channel forinputting local oscillation light.
 16. The optical hybrid circuitaccording to claim 9, further comprising: a light delay circuitconnected one of the channels on the input side of the multimodeinterference coupler; and a 1:2 optical coupler connected to the lightdelay circuit and the other channel on the input side of the multimodeinterference coupler and having one channel on the input side and twochannels on the output side; and wherein the channel on the input sideof the 1:2 optical coupler is an input channel for inputtingdifferential quadrature phase shift keying signal light.
 17. An opticalreceiver comprising: a multimode interference coupler including a pairof input channels provided at positions symmetrical with each other withrespect to a center position in a widthwise direction thereof, a pair offirst output channels neighboring with each other for outputting a pairof first optical signals having an in-phase relationship with eachother, and a pair of second output channels neighboring with each otherfor outputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other, and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals; a photodiode adapted to convert the first optical signals orthe second optical signals outputted from the multimode interferencecoupler and the third optical signals outputted from the 2:2 opticalcoupler into an analog electric signal; an analog-digital conversioncircuit adapted to convert the analog electric signal outputted from thephotodiode into a digital electric signal; and a digital arithmeticcircuit adapted to execute a arithmetic processing using the digitalelectric signal outputted from the analog-digital conversion circuit.18. An optical receiver comprising: an optical hybrid circuit includinga multimode interference coupler including a pair of input channelsprovided at positions symmetrical with each other with respect to acenter position in a widthwise direction thereof, a pair of first outputchannels neighboring with each other for outputting a pair of firstoptical signals having an in-phase relationship with each other, and apair of second output channels neighboring with each other foroutputting a pair of second optical signals having an in-phaserelationship with each other, the multimode interference coupler beingadapted to convert quadrature phase shift keying signal light ordifferential quadrature phase shift keying signal light into the pair offirst optical signals having an in-phase relationship with each otherand the pair of second optical signals having an in-phase relationshipwith each other, and a 2:2 optical coupler connected to the first outputchannels or the second output channels and having two channels on theinput side and two channels on the output side, the 2:2 optical couplerbeing adapted to convert the pair of first optical signals or the pairof second optical signals into a pair of third optical signals having aquadrature phase relationship with the pair of first or second opticalsignals, the multimode interference coupler having an input end of afirst width and an output end of a second width different from the firstwidth such that a phase difference between the pair of first opticalsignals or a phase difference between the pair of second optical signalsbecomes equal to π/2+p*π, p being an integer; a photodiode adapted toconvert the first optical signals or the second optical signalsoutputted from the multimode interference coupler and the third opticalsignals outputted from the 2:2 optical coupler into an analog electricsignal; an analog-digital conversion circuit adapted to convert theanalog electric signal outputted from the photodiode into a digitalelectric signal; and a digital arithmetic circuit adapted to executearithmetic processing using the digital electric signal outputted fromthe analog-digital conversion circuit.
 19. A light receiving methodcomprising: converting, using a multimode interference coupler includinga pair of input channels provided at positions symmetrical with eachother with respect to a center position in a widthwise directionthereof, a pair of first output channels neighboring with each other foroutputting a pair of first optical signals having an in-phaserelationship with each other, and a pair of second output channelsneighboring with each other for outputting a pair of second opticalsignals having an in-phase relationship with each other, quadraturephase shift keying signal light or differential quadrature phase shiftkeying signal light into the pair of first optical signals having anin-phase relationship with each other and the pair of second opticalsignals having an in-phase relationship with each other; converting,using a 2:2 optical coupler connected to the first output channels orthe second output channels and having two channels on the input side andtwo channels on the output side, the pair of first optical signals orthe pair of second optical signals into a pair of third optical signalshaving a quadrature phase relationship with the pair of first or secondoptical signals; and receiving the first optical signals or the secondoptical signals and the third optical signals.
 20. A light receivingmethod comprising: converting, using a multimode interference couplerincluding a pair of input channels provided at positions symmetricalwith each other with respect to a center position in a widthwisedirection thereof, a pair of first output channels neighboring with eachother for outputting a pair of first optical signals having an in-phaserelationship with each other, and a pair of second output channelsneighboring with each other for outputting a pair of second opticalsignals having an in-phase relationship with each other, quadraturephase shift keying signal light or differential quadrature phase shiftkeying signal light into the pair of first optical signals having anin-phase relationship with each other and the pair of second opticalsignals having an in-phase relationship with each other; converting,using a 2:2 optical coupler connected to the first output channels orthe second output channels and having two channels on the input side andtwo channels on the output side, the pair of first optical signals orthe pair of second optical signals into a pair of third optical signalshaving a quadrature phase relationship with the pair of first or secondoptical signals; receiving the first optical signals or the secondoptical signals and the third optical signals; the multimodeinterference coupler having an input end of a first width and an outputend of a second width different from the first width such that a phasedifference between the pair of first optical signals or a phasedifference between the pair of second optical signals becomes equal toπ/2+p*π, p being an integer.